Minor changes to ease bundle development via ST2 (ignore, README fix)

ST2 generates .cache files of snippets and other elements in a package, so it's helpful to ignore these, especially when making changes to this repo and using it as the editor package in the Packages dir.

I removed the .tmBundle directory name extension from the ST2 instructions since package directory names in ST2 follow a particular convention. (See your own ~/Application\ Support/Sublime\ Text 2/Packages for examples.)