Homebrew tap for Gerrit Code Review

To install Gerrit with Homebrew

Install the GerritCodeReview/gerrit tap
```
brew tap GerritCodeReview/gerrit
```
Discover the Gerrit versions available
```
brew search gerrit
```
Install the Gerrit desired version
```
brew install gerrit@3.11.0
```

Gerrit data is stored in the Homebrew's var location, /usr/local/var/gerrit.

Start Gerrit as usual with the gerrit.sh and start argument:

/usr/local/var/gerrit/bin/gerrit.sh start

Stop Gerrit passing the stop argument:

/usr/local/var/gerrit/bin/gerrit.sh stop