document the trust step

README.md

@@ -4,9 +4,14 @@ The Homebrew tap for [margin](https://github.com/priyanshujain/margin) and its s
 
 ```
 brew tap priyanshujain/margin
+brew trust priyanshujain/margin
 brew install --cask margin
 ```
 
+The `trust` step is not optional. Homebrew 6 refuses to load a cask from a tap it has not been told
+to trust, and the error it gives instead does not mention installing anything, so leaving it out of
+the instructions just sends people somewhere confusing.
+
 The casks here are rewritten by each app's release workflow, so a version bump lands within a
 minute of the GitHub release leaving draft. Nothing in this repo is edited by hand except when a
 cask gains or loses a stanza.