Add explanation for extra git theme symbols in README.md

Helps with #1589

README.md

@@ -89,7 +89,7 @@ accompanying README files to learn of what is available.
      window or tab.
 
      ![sorin theme][2]  
-
+     Note that the 'git' module may be required for special symbols to appear, such as those on the right of the above image. Add `'git'` to the list under `zstyle ':prezto:load' pmodule \ ` in your `.zpreztorc` to enable this module.
 ### External Modules
 
   1. By default modules will be loaded from */modules* and */contrib*.