Release Note Parsing for Singular Announcements

For reliability we currently refuse to parse/include the contents of your RELEASES/CHANGELOG files if you're publishing a Singular Announcement (`--tag=my-app-v1.0.0`). We will only do it if you're publishing a Unified Announcement (`--tag=v1.0.0`). Singular announcements just get the auto-generated tables.

This is obviously suboptimal, but we need someone who wants this stuff to tell us how they want it!

Some options:

* Look for a RELEASES in the `my-app` package's dir? (kinda wonky for non-virtual workspaces but that's always true)
* Look for extra-special heading formats in the top-level RELEASES? (does [parse-changelog](https://github.com/taiki-e/parse-changelog) support that?)

Start by tracing how RELEASES/CHANGELOG contents are handled for a singular announcement using --tag=my-app-v1.0.0 versus a unified announcement using --tag=v1.0.0. Check whether parse-changelog supports the proposed top-level heading formats and compare that with looking for RELEASES in the package directory; done requires an agreed behavior for singular announcements.

Written by the indexing model from the issue text.