python: add README (#7191)

plugins/python/README.md

+# python plugin
+
+The plugin adds several aliases for useful [python](https://www.python.org/) commands.
+
+To use it, add `python` to the plugins array of your zshrc file:
+```
+plugins=(... python)
+```
+
+## Aliases
+
+| Command          | Description                                                                     |
+|------------------|---------------------------------------------------------------------------------|
+| `pyfind`         | Finds .py files recursively in the current directory                            |
+| `pyclean [dirs]` | Deletes byte-code and cache files from a list of directories or the current one |
+| `pygrep <text>`  | Looks for `text` in .py files                                                   |