docs: add release process documentation

[happysnaker]

Summary

Closes #434

Adds a RELEASING.md that documents the step-by-step release process for pflag maintainers. The current process relies on institutional knowledge that is easy to forget (as noted in #434).

Content

• Prerequisites (write access, GitHub CLI)
• Step-by-step instructions: pull latest, pick version, tag, create release with auto-generated notes
• Notes on --generate-notes vs manual release notes

[tomasaschan]

For the few releases I've shipped, I've used the UI instead of the GH CLI. That has the advantage that you can open the release notes in draft mode even before choosing a tag, and the tag gets published at the same time as the release. Since the PR list in the release notes is a good indicator for which release number (minor or patch) should be tagged, it's useful to have it available before picking a number.

The fact that the UI doesn't create the tag until the release is published is also useful to ensure we don't have tags without releases published, or need to move tags.

[happysnaker]

Thanks for the feedback! I've updated RELEASING.md to document the GitHub UI workflow as the recommended method first, with the GH CLI as an alternative. The UI approach is now listed as Method 1 since it has the advantages you mentioned (atomic tag+release, PR list before version pick).