Add usage information for prompt

modules/prompt/init.zsh

@@ -4,6 +4,13 @@
 # Authors:
 #   Sorin Ionescu <sorin.ionescu@gmail.com>
 #
+# Usage:
+#   To select a prompt theme, add the following to zshrc, and replace 'name'
+#   with the name of the theme you wish to load. Setting it to random will load
+#   a random theme.
+#
+#     zstyle ':omz:module:prompt' theme 'name'
+#
 
 # Load and run the prompt theming system.
 autoload -Uz promptinit && promptinit