Update README.rst to include instructions for vagrant-sshfs error.
    
    This error 'Block in synced folders: internal error invalid:sshfs'
    is due to missing vagrant-sshfs plugin. Added instructions in
    README.rst when faced in this issue.
    
    Fixes https://pagure.io/pagure/issue/3278
    
    Signed-off by: Mohana Datta Yelugoti <ymdatta@protonmail.com></ymdatta@protonmail.com>