Update README.md

README.md

@@ -1,2 +1,14 @@
-# homebrew-gh
-Homebrew tap for GitHub CLI
+# Homebrew tap for GitHub CLI
+
+First-time:
+
+```sh
+brew tap github/gh
+brew install gh --HEAD
+```
+
+To stay up to date with `gh` development:
+
+```sh
+brew upgrade gh --fetch-HEAD
+```