Document gitreview.username

Document git-review specific options
in the Git configuration system.

Currently we have only gitreview.username,
but another one (gitreview.rebase) is
pending in change I96dfab1af48d8f1592766fea4b97e0f35c03d8a5.

Change-Id: I5b34effee355e0075fa3c2c2acea4429198d7e85