Add RELEASE.md: pgxntool here creates branch, not tag

RELEASE.md

@@ -0,0 +1,51 @@
+# RELEASE.md
+
+This repo follows the shared release process in
+[`../ai/RELEASE.md`](https://github.com/Postgres-Extensions/ai/blob/main/RELEASE.md)
+(pending: that doc is currently on branch `release-process-doc`, not yet
+merged — see [ai#8](https://github.com/Postgres-Extensions/ai/pull/8))
+**except for one thing: this repo's vendored `pgxntool` predates the switch
+to git-tag-based releases.**
+
+## `make tag`/`make dist` create a git *branch*, not a tag
+
+The shared doc's own "Notes / gotchas" section anticipates this exact case:
+
+> `make tag`/`make dist` create a *real* git tag, despite older
+> `pgxntool/README.asc` wording describing the result as a "branch" — check
+> the vendored version's actual behavior if a repo's docs still say
+> "branch."
+
+In this repo, the vendored copy's behavior really does match the old
+wording — it's not just stale docs. `pgxntool/base.mk`'s `tag` target runs:
+
+```make
+tag:
+	git branch $(PGXNVERSION)
+	git push --set-upstream origin $(PGXNVERSION)
+```
+
+There is no `git tag` anywhere in this repo's history (`git tag -l` is
+empty). Every past release is a **branch** instead: `0.1`, `0.1.0`, `0.1.1`,
+`0.2.0` (all verified ancestors of `master` — real historical release
+points, not stale forks). `make dist` archives that branch ref the same way
+the shared doc describes archiving a tag, so distribution `.zip` creation
+itself works the same; only the ref type differs.
+
+Practical consequences when following `../ai/RELEASE.md` here:
+
+- **Step 1 (safety check)** and **step 6 (tag and distribute)**: everywhere
+  the shared doc says "the tag" or "`git tag -l`", read "the release
+  branch" / `git branch -r` instead. There is no tag to compare against.
+- **Idempotency differs.** The shared doc notes `make tag` is idempotent if
+  the tag already points at HEAD. That doesn't hold here: `git branch
+  $(PGXNVERSION)` refuses to create a branch that already exists at all
+  (even if it happens to point at the same commit), so re-running `make
+  tag` for an unchanged version still errors. `make forcetag` (`rmtag` +
+  `tag`) is the only way to redo it, same as the shared doc's guidance for
+  moving a tag.
+- `rmtag` deletes the local and remote **branch**, not a tag.
+
+If this ever gets re-vendored from a current pgxntool release, `make tag`
+will start creating real tags and this file can go away (delete
+`RELEASE.md` and rely on `../ai/RELEASE.md` alone once confirmed).