grails: add README (#7362)

plugins/grails/README.md

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