Add local test instructions

Change-Id: I57069906ce4f44439969c086ac035d05193ed843

README.md

diff --git a/README.md b/README.md
index 4b66d17..50f73d8 100644
--- a/README.md
+++ b/README.md
@@ -32,3 +32,10 @@
 ```sh
 /usr/local/var/gerrit/bin/gerrit.sh stop
 ```
+
+## Testing locally
+
+To test locally, first set the following env variable,
+`export HOMEBREW_NO_INSTALL_FROM_API=1`
+then run
+`brew install --build-from-source --verbose --debug <gerrit_version>.rb`