add readme for grails plugin

plugins/grails/README.md

+# Grails plugin
+
+Adds tab-completion of Grails script names to the command line use of grails.
+Looks for scripts in the following paths:
+
+- `$GRAILS_HOME/scripts`
+- `~/.grails/scripts`
+- `./scripts`
+- `./plugins/*/scripts`
+
+To use it, add `grails` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... grails)
+```