chore: add coding rules

Author: gchoqueux

CODING.md

@@ -3,13 +3,15 @@
 iTowns needs `nodejs` (10+) and `npm` (at least 6.x) installed on the system.
 See the [node website](https://nodejs.org) to install node and npm.
 
-## Coding
+## Coding rules
 
 * Search and use existing methods to avoid duplicate code
 * Avoid instance `class/object` in method (optimization with the garbage collector)
 * Give explicite name to constants
 * Avoid duplicate code and encapsulate to simplify
 * For performance reasons, update objects only when needed
+* Don't add dead code
+* Remove obsolete comments
 
 ## Building