added st3 info to README

Author: colinta

README.md

@@ -7,6 +7,8 @@ Shows diffs - also in an external diff tool - between the current file, or selec
 Installation
 ------------
 
+### Sublime Text 2
+
 1. Using Package Control, install "FileDiffs"
 
 Or:
@@ -20,6 +22,13 @@ Or:
 2. clone this repo
 3. Install keymaps for the commands (see Example.sublime-keymap for my preferred keys)
 
+### Sublime Text 3
+
+1. Open the Sublime Text 2 Packages folder
+2. clone this repo, but use the `st3` branch
+
+       git clone -b st3 [email protected]:colinta/SublimeFileDiffs
+
 Add External Diff Tool
 --------