doc: Create a features folder.

Moved rdma and bd documents to doc/features. Added a new
document on rebalance.

Change-Id: I04269202adc9605754fc29876433c88480b822a3
BUG: 811311
Signed-off-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/5395
Reviewed-by: Anand Avati <avati@redhat.com>
Tested-by: Anand Avati <avati@redhat.com>