Added 'usage' to README file.
Open only on save, added options, updated README.

By default, prompt to open the file only when saving a read-only file (setting g:perforce_open_on_save).
Allow prompt when modifying a read-only file (setting g:perforce_open_on_change).
Merge branch 'master' of github.com:nfvs/vim-perforce

Conflicts:
	README.md