readme: add note about commit tags

CONTRIBUTING.md

@@ -46,4 +46,11 @@
   * Short blocks, if statements and loops on single line are allowed.
   * 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.
+  * 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.
+