Check the root resource for capabilities.

Review Board 2.0 will provide the API capabilities in the root resource,
allowing RBTools to save a request. This makes that change by checking
the root before attempting to fetch the info resource.
Mention installing command-line SCM clients in the windows installation.

Trivial change to mention that graphical frontends like Tortoise are
insufficient, and command-line clients are required.

Fixes bug 3075.