cabal: add README (#7367)

plugins/cabal/README.md

+# Cabal
+
+This plugin provides completion for [Cabal](https://www.haskell.org/cabal/), a build tool for Haskell. It
+also provides a function `cabal_sandbox_info` that prints whether the current working directory is in a sandbox.
+
+To use it, add cabal to the plugins array of your zshrc file:
+```
+plugins=(... cabal)
+```