[DOC release] document rare past release bugfixes

Indicate that almost all `BUGFIX`es should be against `master` with `beta` tag.

Author: EricSchank

CONTRIBUTING.md

@@ -221,7 +221,7 @@ In general bug fixes are pulled into the beta branch. As such, the prefix is: `[
 
 For bugs related to canary features, follow the prefixing rules for features.
 
-For bugs specifically for past releases, tag for that release `[BUGFIX release-1-13]` and submit the PR against the stable branch for that release: `emberjs:stable-1-13`
+The vast majority of bug fixes apply to the current stable or beta releases, so submit your PR against `emberjs:master` with one of the above mentioned BUGFIX tags.  (In the unusual case of a bug fix specifically for a past release, tag for that release `[BUGFIX release-1-13]` and submit the PR against the stable branch for that release: `emberjs:stable-1-13`.)
 
 ### Cleanup