readme: add note about commit tags

CONTRIBUTING.md

@@ -47,3 +47,10 @@
   * Avoid magic numbers.
   * While macros are powerful, it's better to avoid overusing them.
   * If you unsure, try to mimic code style from anywhere else of engine source code.
+* **ANY** commit message should start from declaring a tags, in format:
+  `tag: added some bugs`
+  `tag: subtag: fixed some features`
+  
+  Tags can be any: subsystem, simple feature name or even just a filename, without extension.
+  Just keep them always same, it helps keep history clean and commit messages short.
+