Update instructions on updating vimrc (#614)

README.md

@@ -63,8 +63,10 @@ If you have vim aliased as `vi` instead of `vim`, make sure to either alias it:
 Just do a git rebase!
 
     cd ~/.vim_runtime
+    git reset --hard
+    git clean -d --force
     git pull --rebase
-    python update_plugins.py
+    python update_plugins.py  # use python3 if python is unavailable
 
 
 ## Some screenshots