Note on hotkeys for OS X

Some hotkeys are mentioned that are not, by default, the same on OS X. (e.g. ctrl-enter, ctrl-shift-f, possibly more). 

Made a note of this to help new OS X users find the correct hotkey, rather than thread the document with alternative key mappings.
Merge pull request #1583 from gekkoe/patch-2

Note on hotkeys for OS X