Homebrew: fill in README

Signed-off-by: William Woodruff <[email protected]>

alpha/engagements/2023/Homebrew/README.md

@@ -1 +1,21 @@
+# Alpha Engagement: Build provenance for Homebrew
 
+The purpose of this Alpha engagement is to fund the development of build
+provenance for Homebrew (and specifically `homebrew-core`).
+
+The full technical proposal can be found here:
+[Build Provenance and Code-signing for Homebrew].
+
+[Build Provenance and Code-signing for Homebrew]: https://repos.openssf.org/proposals/build-provenance-and-code-signing-for-homebrew
+
+## Timeline
+
+This engagement started in October 2023.
+
+## Monthly Updates
+
+## Primary Contacts
+
+* William Woodruff - Trail of Bits
+
+## Announcement / News