Added contributing guidelines

Author: cincuranet

CONTRIBUTING.md

@@ -0,0 +1,16 @@
+Contributing
+============
+
+Thanks for considering contributing. Here are some items to consider before starting (in no particular order).
+
+* Check [tracker](http://tracker.firebirdsql.org/browse/DNET) if you want to start on something.
+* Before you start working on something announce your intention on the mailing list (or in comment in tracker).
+* Issue only complete PRs (no WIPs).
+* Make sure your PR doesn't contain unneccessary changes (whitespaces, new lines, ...).
+* Follow existing code formatting/styling (even for new files).
+* Make sure you swept all corners (i.e. all build configurations are fine, works with all Firebird version, etc.).
+* Make sure your changes merge without conflicts.
+* Consider backward compatibility.
+* Don't be afraid to ask (i.e. backward compatibility).
+* Be prepared to do some changes after your PR is reviewed.
+* Consider whether your change is benefit for majority of users (opposite to only a small group).