Mention Git tags. Closes #68.

README.md

@@ -38,6 +38,9 @@ For that to work, you need to export a `GITHUB_TOKEN` environment variable with
 the `repo` scope selected. You can create one
 [here](https://github.com/settings/tokens/new).
 
+GoReleaser uses the latest [Git tag](https://git-scm.com/book/en/v2/Git-Basics-Tagging) of your repository,
+so you need to [create a tag](https://git-scm.com/book/en/v2/Git-Basics-Tagging#Annotated-Tags) first.
+
 This will build `main.go` as `my-binary`, for `Darwin` and `Linux`
 (`amd64` and `i386`), archive the binary and common files as `.tar.gz`,
 and finally, publish a new github release in the `user/repo` repository with