Add deprecation guideline

This commit adds some very basic information about our deprecation
policy.

doc/development/README.md

@@ -173,6 +173,10 @@ Complementary reads:
 
 - [Refactoring guidelines](refactoring_guide/index.md)
 
+## Deprecation guides
+
+- [Deprecation guidelines](deprecation_guidelines/index.md)
+
 ## Documentation guides
 
 - [Writing documentation](documentation/index.md)

doc/development/deprecation_guidelines/index.md

+# Deprecation guidelines
+
+This page includes information about how and when to remove or make breaking
+changes to GitLab features.
+
+## Terminology
+
+It's important to understand the difference between **deprecation** and
+**removal**:
+
+**Deprecation** is the process of flagging/marking/announcing that a feature
+will be removed in a future version of GitLab.
+
+**Removal** is the process of actually removing a feature that was previously
+deprecated.
+
+## When can a feature be deprecated?
+
+A feature can be deprecated at any time, provided there is a viable alternative.
+
+## When can a feature be removed/changed?
+
+See our [Release and Maintenance policy](../../policy/maintenance.md).