Update documentation for #46

[mpcjanssen]

auto-save.txt has some unexpected changes. Is this caused by previous updates to README.md?

[teto]

I would like to understand how it plays with other options like let g:auto_save_write_all_buffers = 0. Will that still work for non focused buffer if they have b:auto_save .
Is let g:auto_save_events = ['FocusLost'] enough to autosave every buffer with b:auto_save for instance