Clarify that any incompatibility is a bug, except for rare exceptions

PullRequest: truffleruby/546

Author: eregon

doc/user/compatibility.md

@@ -1,8 +1,11 @@
 # Compatibility
 
-TruffleRuby aims to be highly compatible with the standard implementation of
+TruffleRuby aims to be fully compatible with the standard implementation of
 Ruby, MRI, version 2.4.4, revision 63013.
 
+Any incompatibility with MRI is considered a bug, except for rare cases detailed below.
+If you find an incompatibility with MRI, please [report](https://github.com/oracle/truffleruby/issues) it to us.
+
 Our policy is to match the behaviour of MRI, except where we do not know how to
 do so with good performance for typical Ruby programs. Some features work but
 will have very low performance whenever they are used and we advise against