doc: CONTRIBUTING.md to document the release process

[skip ci]

Author: flavorjones

CONTRIBUTING.md

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+# Contributing to tailwindcss-rails
+
+This doc is a brief introduction on modifying and maintaining this gem.
+
+## Updating to the latest upstream tailwindcss version
+
+Update `lib/tailwindcss/upstream.rb` with the upstream version.
+
+Run `bundle exec rake clobber` then `bundle exec rake download` to ensure the tailwindcss binaries can be downloaded, and that you have the correct versions on local disk.
+
+## Cutting a release
+
+- bump the version
+  - [ ] update `lib/tailwindcss/version.rb`
+  - [ ] update `CHANGELOG.md`
+  - [ ] commit and create a git tag
+- build the native gems:
+  - [ ] `bundle exec rake clobber` to clean up possibly-old tailwindcss executables
+  - [ ] `bundle exec rake package`
+- push
+  - [ ] `for g in pkg/*.gem ; do gem push $g ; done`
+  - [ ] `git push`
+- announce
+  - [ ] create a release at https://github.com/rails/tailwindcss-rails/releases