Add documentation for completion

Fixes #100

modules/completion/README.md

@@ -0,0 +1,19 @@
+Completion
+==========
+Load additional completion files not built in *zsh* but provided by the
+[zsh-completions][1] project.
+
+Note to the developers
+----------------------
+No modules should only add a completion file; if you feel like your completion
+file is important, contribute to the [zsh-completions][1] project directly.
+
+This module will be updated regularly to provide all possible completions.
+
+Authors
+-------
+*The authors of this module should be contacted via the github bug tracker.*
+
+ - [Sorin Ionescu](/sorin-ionescu)
+
+[1]: /zsh-users/zsh-completions