Add README for thor and pip plugins (#8185)

plugins/pip/README.md

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+# pip plugin
+
+This plugin adds completion for [pip](https://pip.pypa.io/en/latest/),
+the Python package manager.
+
+To use it, add `pip` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... pip)
+```
+
+## pip cache
+
+The pip plugin caches the names of available pip packages from the PyPI index.
+To trigger the caching process, try to complete `pip install`,
+or you can run `zsh-pip-cache-packages` directly.
+
+To reset the cache, run `zsh-pip-clear-cache` and it will be rebuilt next
+the next time you autocomplete `pip install`.

plugins/thor/README.md

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+# Thor plugin
+
+This plugin adds completion for [Thor](http://whatisthor.com/), 
+a ruby toolkit for building powerful command-line interfaces.
+
+To use it, add `thor` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... thor)
+```