Fix documentation to correctly indicate how to add commands

[markbirbeck]

The documentation doesn't explain how to add commands. This was intentional since I thought it was outside the scope of a package since it's something that's required by all packages.

However, it's probably needlessly terse!

Note also that this has become even more confusing (see #59) than it was before because ST has changed how user keyboard mappings are added. (They now use OS specific files.)