Add release notes for rez suites

It would be great for rez suite releases to accept release notes from the user generating the suite. Otherwise suites can change without reason and it's a little difficult to discern what people were trying to do.

No files, tests, or entry points are named in the issue. Start by locating the code that creates and releases rez suites, then trace how user-supplied suite metadata is stored; done means a suite generator can provide release notes and released suites preserve and expose them so changes have context.

Written by the indexing model from the issue text.