Updated documentation

README.md

@@ -200,6 +200,10 @@ function prompt_callback {
 }
 ```
 
+- There are two helper functions that can be used within `prompt_callback`:
+    - `gp_set_window_title <String>` - sets the window title to the given string (should work for XTerm type terminals like in OS X or Ubuntu)
+    - `gp_truncate_pwd` - a function that returns the current PWD truncated to fit the current terminal width
+
 - If you want to show the git prompt only if you are in a git repository you
   can set ``GIT_PROMPT_ONLY_IN_REPO=1`` before sourcing the gitprompt script