JetBrains: Document dependency on the "git" command

Created by: vdavid

The git command dependency not being documented caused a problem with a client here: https://github.com/sourcegraph/sourcegraph/issues/40452

We should mention this dependency in the plugin's README.md, plus the plugin page in the JetBrains Marketplace.

/cc @muratsu @jjinnii @ryankscott