docs: document the macOS Gatekeeper quarantine fix for lc (#36)

## Summary

Closes #35.

`lc` (the actual Burrito binary, unsigned/unnotarized) gets blocked by
macOS Gatekeeper after download - the wrapper scripts (`lcls`, etc.) are
plain shell so they're unaffected, only the compiled binary itself.
Documents the `xattr -d com.apple.quarantine` fix right after the
download instructions in the Readme.

## Test plan

- [x] `asciidoctor -o /dev/null Readme.adoc` renders clean

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

Author: bougyman

Readme.adoc

@@ -56,6 +56,14 @@ $ sudo mv lc/* /usr/local/bin/
 
 NOTE: A Homebrew tap is planned but not yet available.
 
+On macOS, Gatekeeper blocks `lc` itself (the wrapper scripts are plain shell,
+so they're unaffected) since it isn't signed/notarized yet:
+
+[source,sh]
+----
+$ xattr -d com.apple.quarantine /usr/local/bin/lc
+----
+
 === install.sh (builds from source, for machines without Homebrew)
 
 [source,sh]