add prompt documentation

modules/prompt/README.md

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+Prompt
+======
+
+Adds additional prompts and initializes the prompt system.
+
+A list of available prompts can be found by running `prompt -l`.
+
+To test a prompt, simply run `prompt prompt_name_here`.
+
+.zimrc Configuration
+--------------------
+
+  * `zprompt_theme='eriner'` change 'eriner' to the name of your desired prompt. This will set it as you default prompt.