cpanm: add README (#7377)

plugins/cpanm/README.md

+# Cpanm
+
+This plugin provides completion for [Cpanm](https://github.com/miyagawa/cpanminus) ([docs](https://metacpan.org/pod/App::cpanminus)).
+ 
+To use it add cpanm to the plugins array in your zshrc file.
+ 
+ ```bash
+plugins=(... cpanm)
+```

plugins/cpanm/_cpanm

@@ -6,9 +6,6 @@
 #
 # Current supported cpanm version: 1.4000 (Tue Mar  8 01:00:49 PST 2011)
 #
-# The latest code is always located at:
-#   https://github.com/rshhh/cpanminus/blob/master/etc/_cpanm
-#
 
 local arguments curcontext="$curcontext"
 typeset -A opt_args