glassfish: add README

plugins/glassfish/README.md

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+# glassfish
+
+The glassfish plugin adds completion for the `asadmin` utility, a command to manage
+[Oracle GlassFish](https://docs.oracle.com/cd/E18930_01/html/821-2416/giobi.html) servers.
+
+To use it, add `glassfish` to the plugins array in your zshrc file:
+```zsh
+plugins=(... glassfish)
+```