Add developer docs for how to add a licensed feature

doc/development/README.md

@@ -33,6 +33,7 @@ comments: false
 - [Sidekiq guidelines](sidekiq_style_guide.md) for working with Sidekiq workers
 - [Working with Gitaly](gitaly.md)
 - [Manage feature flags](feature_flags.md)
+- [Licensed feature availability](licensed_feature_availability.md)
 - [View sent emails or preview mailers](emails.md)
 - [Shell commands](shell_commands.md) in the GitLab codebase
 - [`Gemfile` guidelines](gemfile.md)

doc/development/licensed_feature_availability.md

+# Licensed feature availability
+
+As of GitLab 9.4, we've been supporting a simplified version of licensed 
+feature availability checks via `lib/license.rb`, both for 
+on-premise or GitLab.com plans and features.
+
+## Restricting features scoped by namespaces or projects
+
+GitLab.com plans are persisted on user groups and namespaces, therefore, if you're adding a
+feature such as [Related issues](../user/project/issues/related_issues.md) or 
+[Service desk](../user/project/service_desk.md), 
+it should be restricted on namespace scope.
+
+1. Add the feature symbol on `EES_FEATURES`, `EEP_FEATURES` or `EEU_FEATURES` constants in 
+  `lib/license.rb`. Note on `ee/app/models/ee/namespace.rb` that _Bronze_ GitLab.com 
+  features maps to on-premise _EES_, _Silver_ to _EEP_ and _Gold_ to _EEU_.
+2. Check using:
+
+```ruby
+project.feature_available?(:feature_symbol)
+```
+
+## Restricting global features (instance)
+
+However, for features such as [Geo](../gitlab-geo/README.md) and 
+[Load balancing](../administration/database_load_balancing.md), which cannot be restricted 
+to only a subset of projects or namespaces, the check will be made directly in 
+the instance license.
+
+1. Add the feature symbol on `EES_FEATURES`, `EEP_FEATURES` or `EEU_FEATURES` constants in 
+  `lib/license.rb`.
+2. Add the same feature symbol to `GLOBAL_FEATURES`
+3. Check using:
+
+```ruby
+License.feature_available?(:feature_symbol)
+```