Add .gitreview file

`git-review` is a command line tool for interacting with Gerrit.
Some IDEs offer support for this tool, either natively or through
plugins. The tool relies on a configuration file, .gitreview. Adding
this configuration file to our repositories simplifies initial setup
for contributors that use the tool directly or through their IDE of
choice.

The configuration file adds a remote called 'gerrit'. This is the
default for the tool, and also the name set for
codereview.qt-project.org by Qt's `init-repository` script. Thus,
the configuration should work seamlessly alongside other repository
helpers.

Task-number: QTBUG-132604
Change-Id: If16a5402b012637aa298df862f03006f7deb6984
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>