Added -FF flag to rsync invocation in synchronize

The `-FF` option causes rsync to look for files in the source directory named `.rsync-filter` and uses them to filter directories underneath them. If no `.rsync-filter` files are found, the behavior is identical to the command run without the -FF option. This flag does not sync the .rsync-filter files themselves.

This change should be backwards compatible and not produce surprising behavior for users, since they are unlikely to create `.rsync-filter` files unintentionally.
Added note about .rsync-filter files, with example
Merge pull request #5051 from deontologician/patch-1

Added -FF flag to rsync invocation in synchronize