Add CONTRIBUTING page (#360)

* Add CONTRIBUTING page

* Add Gemfile examples

---------

Co-authored-by: Andy Waite <[email protected]>

Author: andyw8

CONTRIBUTING.md

@@ -0,0 +1,33 @@
+# CONTRIBUTING
+
+Bug reports and pull requests are welcome on GitHub at https://github.com/Shopify/ruby-lsp-rails. This project is
+intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the
+[Contributor Covenant](https://github.com/Shopify/ruby-lsp-rails/blob/main/CODE_OF_CONDUCT.md) code of conduct.
+
+# Developing on Ruby LSP
+
+For general information about developing, refer to the to the [documentation](https://github.com/Shopify/ruby-lsp/blob/main/CONTRIBUTING.md#debugging-with-vs-code) for Ruby LSP itself.
+
+### Manually testing a change
+
+The repo includes a dummy Rails app in `test/dummy`. If you're developing a feature, you can add new code to it for testing.
+
+To test with a real Rails application, you can add a Gemfile entry for `ruby-lsp-rails` and point it to your local checkout:
+
+```ruby
+gem "ruby-lsp-rails", path: "../ruby-lsp-rails"
+```
+
+Or to a branch in your fork:
+
+```ruby
+gem "ruby-lsp-rails", github: "USERNAME/ruby-lsp-rails", branch: "your_branch"
+```
+
+### Running the test suite
+
+The test suite can be executed by running
+
+```shell
+bin/rails test
+```

README.md

@@ -37,9 +37,7 @@ When extension is stopped (e.g. by quitting the editor), the server instance is
 
 ## Contributing
 
-Bug reports and pull requests are welcome on GitHub at https://github.com/Shopify/ruby-lsp-rails. This project is
-intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the
-[Contributor Covenant](https://github.com/Shopify/ruby-lsp-rails/blob/main/CODE_OF_CONDUCT.md) code of conduct.
+See [CONTRIBUTING.md](https://github.com/Shopify/ruby-lsp-rails/blob/main/CONTRIBUTING.md)
 
 ## License