Add README for thor and pip plugins (#8185)

plugins/thor/README.md

@@ -0,0 +1,10 @@
+# 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)
+```