Clarify in README that bugs are filed in bugzilla

Also link to CONTRIBUTING.md in case folks can't find it.

Author: chutten

README.md

@@ -10,6 +10,7 @@ code for various integrations, linting and coverage testing.
 
 ## Documentation
 
+- [How to Contribute](https://github.com/mozilla/glean_parser/blob/main/CONTRIBUTING.md). Please file bugs in [bugzilla](https://bugzilla.mozilla.org/enter_bug.cgi?assigned_to=nobody%40mozilla.org&bug_ignored=0&bug_severity=normal&bug_status=NEW&cf_fission_milestone=---&cf_fx_iteration=---&cf_fx_points=---&cf_status_firefox65=---&cf_status_firefox66=---&cf_status_firefox67=---&cf_status_firefox_esr60=---&cf_status_thunderbird_esr60=---&cf_tracking_firefox65=---&cf_tracking_firefox66=---&cf_tracking_firefox67=---&cf_tracking_firefox_esr60=---&cf_tracking_firefox_relnote=---&cf_tracking_thunderbird_esr60=---&product=Data%20Platform%20and%20Tools&component=Glean%3A%20SDK&contenttypemethod=list&contenttypeselection=text%2Fplain&defined_groups=1&flag_type-203=X&flag_type-37=X&flag_type-41=X&flag_type-607=X&flag_type-721=X&flag_type-737=X&flag_type-787=X&flag_type-799=X&flag_type-800=X&flag_type-803=X&flag_type-835=X&flag_type-846=X&flag_type-855=X&flag_type-864=X&flag_type-916=X&flag_type-929=X&flag_type-930=X&flag_type-935=X&flag_type-936=X&flag_type-937=X&form_name=enter_bug&maketemplate=Remember%20values%20as%20bookmarkable%20template&op_sys=Unspecified&priority=P3&&rep_platform=Unspecified&status_whiteboard=%5Btelemetry%3Aglean-rs%3Am%3F%5D&target_milestone=---&version=unspecified).
 - [User documentation for Glean](https://mozilla.github.io/glean/).
 - [`glean_parser` developer documentation](https://mozilla.github.io/glean_parser/).