Point out validate.sh in CONTRIBUTING.md

Author: 9999years

CONTRIBUTING.md

@@ -59,6 +59,9 @@ $ cabal build cabal-tests            # etc...
 
 ## Running tests
 
+There are two ways to run tests: in CI with GitHub actions and locally with
+`./validate.sh`.
+
 ### Using GitHub Actions.
 
 If you are not in a hurry, the most convenient way to run tests on Cabal
@@ -115,6 +118,9 @@ or continuous integration breakage; please file a bug.
 
 ### Running tests locally.
 
+The [`./validate.sh`](./validate.sh) script runs all the test suites. It takes
+various options to restrict the test suites it runs; use `--help` to list them.
+
 To run tests locally with `cabal`, you will need to know the
 name of the test suite you want.  Cabal and cabal-install have
 several.  Also, you'll want to read [Where are my build products?](http://cabal.readthedocs.io/en/latest/nix-local-build.html#where-are-my-build-products)