Update README with usage instructions

README.md

@@ -3,10 +3,30 @@
 This repository contains packages patched to use the `Hooks` build-type.
 It can be used as an overlay much like `head.hackage`.
 
-
 ## Usage information
 
-TODO.
+To allow `cabal-install` to pull patches from this repository, you should
+add the following to your `cabal.project` or `cabal.project.local` file:
+
+```
+repository hooks-setup-testing.ghc.haskell.org
+   url: https://mpickering.gitlab.haskell.org/hooks-setup-testing/
+   secure: True
+   key-threshold: 3
+   root-keys:
+       46b3ab3433bf273303a01b658a9be7f166e525c07f1ad6598e9d1d7c4ea0bf46
+       f2cd708c5a68b805f0e150d75811c16eb1fc237be52a5584fb5b3b8e1e990ac9
+       80bda11a92009dcf3ff70696475f1564b55b8d6796e0588929193a45022b8509
+       29b00e90e759b822be58c3dc43c77fa4aba0b0fa6a06f611e26ba77f00b77feb
+       4e083c2c26f3a9231560eb8b2c0a88ab6b661039fce3165cd2936514d771fb9c
+       b20a85388d60c2d74ad52240dc23e7dea3bb21bfb1b9ffdfbe6fe3a771ca168b
+
+active-repositories: hackage.haskell.org, hooks-setup-testing.ghc.haskell.org:override
+```
+
+The use of `:override` forces `cabal`'s constraint solver to pick versions of
+libraries that have corresponding patches in this overlay repository whenever
+possible.
 
 ## Development information