Update CONTRIBUTING.md (add list of valid prefix commit message)

CONTRIBUTING.md

-# How to contribute to Orfeo ToolBox
+# How to contribute to Orfeo ToolBox ?
 
 Thank you for taking the time to contribute to OTB! This document will guide you
 through the workflow and best practices you need to know to send your
@@ -72,6 +72,16 @@ commit message so that GitLab can [crosslink it](https://docs.gitlab.com/ce/user
 You can prefix your commit message with an indicating flag (DOC, BUG, PKG,
 TEST, SuperBuild, etc.) or the name of the module you are changing.
 
+Standard prefixes for OTB commit messages:
+
+    BUG: Fix for runtime crash or incorrect result
+    COMP: Compiler error or warning fix
+    DOC: Documentation change
+    ENH: New functionality
+    PERF: Performance improvement
+    STYLE: No logic impact (indentation, comments)
+    WIP: Work In Progress not ready for merge
+
 For example, here are some good commit messages:
 
     BUG: #1701 Warn users if parameter string is unset