mtree: Update mtree flags in README file

• Add -b (suppress blank lines before directories).
• The equivalent of -i in fmtree is -j in mtree (nmtree) (indent the output 4 spaces).
• Add -F freebsd9 compatibility flavor (print the closing .. at the end).

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/912