Add sublime-text-3 gitignore

This gitignore is used to make a repo with sublime text 3 package preferences stored in
{sublime-text-3}/Packages/User/

https://www.sublimetext.com/3

The .gitignore files are listed here

https://packagecontrol.io/docs/syncing

I added GitHub.sublime-settings to the .gitignore since its a config file added
from a popular github package that contains sensible information about your account