Add Readme.md for supervisor.

Sync completion with original source.

plugins/supervisor/README.md

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+# supervisor plugin
+
+This plugin adds tab-completion for supervisord/supervisorctl in [Supervisor](http://supervisord.org/).
+
+Supervisor is a client/server system that allows its users to monitor and control a number of processes on UNIX-like operating systems.
+
+To use it, add `supervisor` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... supervisor)
+```
+
+These scripts are from [zshcompfunc4supervisor](https://bitbucket.org/hhatto/zshcompfunc4supervisor).