docs: write the tap README

README.md

@@ -1,6 +1,43 @@
 # Mergify Homebrew Tap
 
-Homebrew tap for [Mergify](https://mergify.com) command-line tools.
+Homebrew tap for the [Mergify](https://mergify.com) CLI — stacked pull
+requests, CI insights, merge queue, scheduled freezes, and config management.
+
+## Install
+
+```shell
+brew install mergifyio/tap/mergify-cli
+```
+
+The fully-qualified name taps and installs in one step. Then run `mergify --help`.
+
+> **Homebrew 6.0 tap trust.** Since Homebrew 6.0, third-party taps must be
+> trusted before their code runs. Installing by the **fully-qualified** name
+> above trusts just this formula, so the command works as-is. To instead add
+> the tap and install by short name, trust it once:
+>
+> ```shell
+> brew tap mergifyio/tap
+> brew trust mergifyio/tap
+> brew install mergify-cli
+> ```
+>
+> See [Homebrew Tap Trust](https://docs.brew.sh/Tap-Trust).
+
+## Updating
+
+```shell
+brew upgrade mergify-cli
+```
+
+Manage it with Homebrew, **not** `mergify self-update` — self-update overwrites
+the Homebrew-managed binary and is reverted on the next `brew upgrade`.
+
+## Maintainers
+
+The formula is `Formula/mergify-cli.rb`. To ship a new CLI version, bump the
+`RELEASE` constant and the four `sha256`s, then open a PR — the merge queue
+tests all four platforms and merges on green.
 
 ## License