rebar: add README (#8198)

plugins/rebar/README.md

@@ -0,0 +1,9 @@
+# rebar plugin
+
+This plugin adds completions for the [rebar](https://www.rebar3.org/) Erlang build tool.
+
+To use it, add `rebar` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... rebar)
+```