Add changeset tag to custom publishing section of README

[mholta]

Why is this pull request needed?

Based on #269.

What does this pull request change?

Add simple example of custom publishing workflow that produces GitHub Releases and Git tags.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 9b099e6

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[gtjamesa]

This PR definitely would have saved me reading through the source code

[helloint]

why is this not being merged?