Document "No gems fetched from git repositories" policy [ci skip]

doc/development/README.md

@@ -18,6 +18,7 @@
 - [Frontend guidelines](frontend.md)
 - [SQL guidelines](sql.md) for working with SQL queries
 - [Sidekiq guidelines](sidekiq_style_guide.md) for working with Sidekiq workers
+- [`Gemfile` guidelines](gemfile.md)
 
 ## Process
 

doc/development/gemfile.md

+# `Gemfile` guidelines
+
+When adding a new entry to `Gemfile` or upgrading an existing dependency pay
+attention to the following rules.
+
+## No gems fetched from git repositories
+
+We do not allow gems that are fetched from git repositories. All gems have
+to be available in the RubyGems index. We want to minimize external build
+dependencies and build times.
+
+## License compliance
+
+Refer to [licensing guidelines](licensing.md) for ensuring license compliance.