Add convenience shortcuts to go to source code

[robert-stuttaford]

As it says on the tin :-)

One to go to source, and one to go to source and keep the cheatsheet open.

Thanks for a rocking addition to Emacs!

[krisajenkins]

Thanks Robert!

There's already a shortcut for 'open source' (C-e), so I've added a note to the README. I've also added a note about C-h m which will show you all the shortcuts. The cheatsheet's built on top of Helm, so there are a fair few I get for free. :-)

For 'open source, keep open', you can do TAB C-n C-z at the moment. I'll look into something better when I get chance. :-)

[robert-stuttaford]

Awesome, thanks, Kris!