add simple CONTRIBUTING.md

Fixes #708

[CI SKIP]

CONTRIBUTING.md

@@ -0,0 +1,22 @@
+Reporting bugs
+==============
+
+* When reporting a bug:
+    * Please ensure that you are on the latest version in case the bug you
+      are reporting is already fixed.
+    * Please include the following information:
+        * OS/distribution version
+        * `/znc version`
+    * If you are reporting a crash, please see [the debugging page] on
+      wiki.znc.in.
+
+[The debugging page]:http://wiki.znc.in/Debugging
+
+Code changes
+============
+
+* Follow the exact same conventions and style of the file you change.
+* For deciding which branch to pull request, please see [the branches page]
+  on wiki.znc.in.
+
+[The branches page]:http://wiki.znc.in/Branches