README.asc: make tag/make dist create a real git tag, not a branch

[jnasbyupgrade]

`pgxntool/README.asc`'s "tag" section says:

`make tag` will create a git branch for the current version of your extension...

This is stale. Current `base.mk` (`tag`/`forcetag`/`rmtag` targets) creates and
pushes a real annotated git tag (`git tag $(PGXNVERSION)`, `git push origin
$(PGXNVERSION)`), not a branch. `make dist` then does `git archive` at that tag.

Please update the docs to describe the actual (tag-based) behavior.

Found while writing a release-process doc for extension_tools/extension_drop:
https://github.com/Postgres-Extensions/extension_tools