Add a blurb about filing Issues

Since this document is linked to from "New Issue" page, it's
helpful to include "rules" for bug reports first and place all
the info about contributions later.

CONTRIBUTING.md

@@ -1,3 +1,14 @@
+# Filing Issues
+
+## Potential bugs
+
+Please include a way for developers to reproduce the problem. A small program
+that demonstrates a problem is best.
+
+Describe the behaviour you're observing and how it differs from expectations.
+Include the version of the compiler you're using (run `perl6 -v`) as well as
+the type and version of the operating system (e.g. `Windows 10`)
+
 # How to Contribute to Rakudo Perl 6
 
 Contributions to Rakudo are very welcome.