[Fix #261] Add documentation for writing prompt themes

modules/prompt/README.md

@@ -12,13 +12,101 @@ a random theme.
 
     zstyle ':prezto:module:prompt' theme 'name'
 
+Theming
+-------
+
+A prompt theme is an autoloadable function file with a special name,
+`prompt_name_setup`, placed anywhere in `$fpath`, but for the purpose of this
+project, themes **should** be placed in the *modules/prompt/functions*
+directory.
+
+### Theme Functions
+
+There are three theme functions, a setup function, a help function, and
+a preview function. The setup function **must** always be defined. The help
+function and the preview functions are optional.
+
+#### prompt_name_setup
+
+This function is called by the `prompt` function to install the theme. This
+function may define other functions as necessary to maintain the prompt,
+including a function that displays help or a function used to preview it.
+
+**Do not call this function directly.**
+
+The most basic example of this function can be seen bellow.
+
+    function prompt_name_setup {
+      PROMPT='%m%# '
+      RPROMPT=''
+    }
+
+#### prompt_name_help
+
+If the `prompt_name_setup` function is customizable via parameters, a help
+function **should** be defined. The user will access it via `prompt -h name`.
+
+The most basic example of this function can be seen bellow.
+
+    function prompt_name_help {
+      cat <<EOH
+    This prompt is color-scheme-able. You can invoke it thus:
+
+      prompt theme [<color1>] [<color2>]
+
+    where the color is for the left-hand prompt.
+    EOH
+    }
+
+#### prompt_name_preview
+
+If the `prompt_name_setup` function is customizable via parameters, a preview
+function **should** be defined. The user will access it via `prompt -p name`.
+
+The most basic example of this function can be seen bellow.
+
+    function prompt_name_preview {
+      if (( $# > 0 )); then
+        prompt_preview_theme theme "$@"
+      else
+        prompt_preview_theme theme red green blue
+        print
+        prompt_preview_theme theme yellow magenta black
+      fi
+    }
+
+### Hook Functions
+
+There are many Zsh [hook][2] functions, but mostly the *precmd* hook will be
+used.
+
+#### prompt_name_precmd
+
+This hook is called before the prompt is displayed and is useful for getting
+information to display in a prompt.
+
+When calling functions to get information to display in a prompt, do not assume
+that all the dependencies have been loaded. Always check for the availability of
+a function before you calling it.
+
+**Do not register hook functions. They will be registered by the `prompt` function.**
+
+The most basic example of this function can be seen bellow.
+
+    function prompt_name_precmd {
+      if (( $+functions[git-info] )); then
+        git-info
+      fi
+    }
+
 Authors
 -------
 
-*The authors of this module should be contacted via the [issue tracker][2].*
+*The authors of this module should be contacted via the [issue tracker][3].*
 
   - [Sorin Ionescu](https://github.com/sorin-ionescu)
 
 [1]: http://zsh.sourceforge.net/Doc/Release/User-Contributions.html#Prompt-Themes
-[2]: https://github.com/sorin-ionescu/prezto/issues
+[2]: http://zsh.sourceforge.net/Doc/Release/Functions.html#Hook-Functions
+[3]: https://github.com/sorin-ionescu/prezto/issues