Add some docs about useful step before filing a PR (#151)

* Add a document about contributing

It is shamelessly stolen from opentelemetry-python and edited to be
relevant to opentelemetry-go. This file should explain some less
obvious things about the development of the project, like running
linters and regenerating the files.

* Force the use of go modules

Until go 1.13 modules were disabled by default if the repository was
inside GOPATH. This was causing issues when running `make precommit`
for the first time, as it could not find sources for any of the tools
used in `make precommit` under `${GOPATH}/src`. With setting the
`GO111MODULE` environment variable to `on`, we tell `go` to behave
differently, just like go 1.13 does by default.

CONTRIBUTING.md

@@ -0,0 +1,136 @@
+# Contributing to opentelemetry-go
+
+The Go special interest group (SIG) meets regularly. See the
+OpenTelemetry
+[community](https://github.com/open-telemetry/community#golang-sdk)
+repo for information on this and other language SIGs.
+
+See the [public meeting
+notes](https://docs.google.com/document/d/1A63zSWX0x2CyCK_LoNhmQC4rqhLpYXJzXbEPDUQ2n6w/edit#heading=h.9tngw7jdwd6b)
+for a summary description of past meetings. To request edit access,
+join the meeting or get in touch on
+[Gitter](https://gitter.im/open-telemetry/opentelemetry-go).
+
+## Development
+
+There are some generated files checked into the repo. To make sure
+that the generated files are up-to-date, run `make` (or `make
+precommit` - the `precommit` target is the default).
+
+The `precommit` target also fixes the formatting of the code and
+checks the status of the go module files.
+
+If after running `make precommit` the output of `git status` contains
+`nothing to commit, working tree clean` then it means that everything
+is up-to-date and properly formatted.
+
+## Pull Requests
+
+### How to Send Pull Requests
+
+Everyone is welcome to contribute code to `opentelemetry-go` via
+GitHub pull requests (PRs).
+
+To create a new PR, fork the project in GitHub and clone the upstream
+repo:
+
+```sh
+$ go get -d go.opentelemetry.io
+```
+
+(This may print some warning about "build constraints exclude all Go
+files", just ignore it.)
+
+This will put the project in `${GOPATH}/src/go.opentelemetry.io`. You
+can alternatively use `git` directly with:
+
+```sh
+$ git clone https://github.com/open-telemetry/opentelemetry-go
+```
+
+(Note that `git clone` is *not* using the `go.opentelemetry.io` name -
+that name is a kind of a redirector to GitHub that `go get` can
+understand, but `git` does not.)
+
+This would put the project in the `opentelemetry-go` directory in
+current working directory.
+
+Enter the newly created directory and add your fork as a new remote:
+
+```sh
+$ git remote add <YOUR_FORK> git@github.com:<YOUR_GITHUB_USERNAME>/opentelemetry-go
+```
+
+Check out a new branch, make modifications, run linters and tests, and
+push the branch to your fork:
+
+```sh
+$ git checkout -b <YOUR_BRANCH_NAME>
+# edit files
+$ make precommit
+$ make test
+$ git add -p
+$ git commit
+$ git push <YOUR_FORK> <YOUR_BRANCH_NAME>
+```
+
+Open a pull request against the main `opentelemetry-go` repo.
+
+### How to Receive Comments
+
+* If the PR is not ready for review, please put `[WIP]` in the title,
+  tag it as `work-in-progress`, or mark it as
+  [`draft`](https://github.blog/2019-02-14-introducing-draft-pull-requests/).
+* Make sure CLA is signed and CI is clear.
+
+### How to Get PRs Merged
+
+A PR is considered to be **ready to merge** when:
+
+* It has received two approvals from Collaborators/Maintainers (at
+  different companies).
+* Major feedbacks are resolved.
+* It has been open for review for at least one working day. This gives
+  people reasonable time to review.
+* Trivial change (typo, cosmetic, doc, etc.) doesn't have to wait for
+  one day.
+* Urgent fix can take exception as long as it has been actively
+  communicated.
+
+Any Collaborator/Maintainer can merge the PR once it is **ready to
+merge**.
+
+## Design Choices
+
+As with other OpenTelemetry clients, opentelemetry-go follows the
+[opentelemetry-specification](https://github.com/open-telemetry/opentelemetry-specification).
+
+It's especially valuable to read through the [library
+guidelines](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/library-guidelines.md).
+
+### Focus on Capabilities, Not Structure Compliance
+
+OpenTelemetry is an evolving specification, one where the desires and
+use cases are clear, but the method to satisfy those uses cases are
+not.
+
+As such, Contributions should provide functionality and behavior that
+conforms to the specification, but the interface and structure is
+flexible.
+
+It is preferable to have contributions follow the idioms of the
+language rather than conform to specific API names or argument
+patterns in the spec.
+
+For a deeper discussion, see:
+https://github.com/open-telemetry/opentelemetry-specification/issues/165
+
+## Style Guide
+
+* Make sure to run `make precommit` - this will find and fix the code
+  formatting.
+
+## Become an Approver or a Maintainer
+
+See the [community membership document in OpenTelemetry community
+repo](https://github.com/open-telemetry/community/blob/master/community-membership.md).

Makefile

@@ -1,4 +1,6 @@
-ALL_PKGS := $(shell go list ./...)
+ALL_PKGS := $(shell GO111MODULE=on go list ./...)
+
+export GO111MODULE=on
 
 EXAMPLES := \
 	./example/basic \
@@ -55,7 +57,7 @@ circle-ci: precommit test-clean-work-tree test-with-coverage test-386 examples
 test-clean-work-tree:
 	@if ! git diff --quiet; then \
 	  echo; \
-	  echo "Working tree is not clean"; \
+	  echo 'Working tree is not clean, did you forget to run "make precommit"?'; \
 	  echo; \
 	  git status; \
 	  exit 1; \

README.md

@@ -6,6 +6,10 @@
 
 The Go [OpenTelemetry](https://opentelemetry.io/) client.
 
+## Contributing
+
+See the [contributing file](CONTRIBUTING.md).
+
 ## TODO
 
 TODO