cifs: update README to include details about 'fsc' option

  gid		Set the default gid for inodes (similar to above).

  file_mode     If CIFS Unix extensions are not supported by the server

		this overrides the default mode for file inodes.

  fsc		Enable local disk caching using FS-Cache (off by default). This

  		option could be useful to improve performance on a slow link,

		heavily loaded server and/or network where reading from the

		disk is faster than reading from the server (over the network).

		This could also impact scalability positively as the

		number of calls to the server are reduced. However, local

		caching is not suitable for all workloads for e.g. read-once

		type workloads. So, you need to consider carefully your

		workload/scenario before using this option. Currently, local

		disk caching is functional for CIFS files opened as read-only.

  dir_mode      If CIFS Unix extensions are not supported by the server 

		this overrides the default mode for directory inodes.

  port		attempt to contact the server on this tcp port, before