Update document and operate-on-number key bindings

Signed-off-by: Tu Do <tuhdo1710@gmail.com>
Merge pull request #554 from tuhdo/master

Update document and operate-on-number key bindings
Move the prelude tips for key-chord-mode to it's module file.
Merge pull request #555 from hbin/prelude-tips-for-key-chord-mode

Move the prelude tips for key-chord-mode to its module file.
Merge remote-tracking branch 'upstream/master'