Add Homebrew documentation for versioning

- Introduced a new document, HOMEBREW.md, detailing how to set the version for the Homebrew formula.
- Included instructions for injecting version information at build time to ensure the binary reports the correct version instead of 'dev'.

Author: miguelmartens

docs/HOMEBREW.md

@@ -0,0 +1,14 @@
+# Homebrew tap — version ldflag
+
+The binary shows its version with `cowardly version` or `cowardly -v`. To avoid showing `dev`, the **Homebrew formula must inject the version** at build time.
+
+In the tap repo [miguelmartens/homebrew-cowardly](https://github.com/miguelmartens/homebrew-cowardly), ensure the formula passes the version in ldflags:
+
+```ruby
+def install
+  ldflags = "-s -w -X main.Version=v#{version}"
+  system "go", "build", *std_go_args(ldflags: ldflags), "./cmd/cowardly"
+end
+```
+
+So the built binary reports e.g. `Cowardly version: v0.2.5` instead of `Cowardly version: dev`. Homebrew’s `version` is taken from the tag in the tarball URL (e.g. `v0.2.5` → `version` is `0.2.5`), hence `v#{version}`.