* 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.
