Point out validate.sh more prominently in CONTRIBUTING.md

Author: 9999years

CONTRIBUTING.md

@@ -61,6 +61,8 @@ $ cabal build cabal-tests            # etc...
 Running tests
 -------------
 
+**TL;DR:** `./validate.sh` runs all the test suites.
+
 **Using GitHub Actions.**
 If you are not in a hurry, the most convenient way to run tests on Cabal
 is to make a branch on GitHub and then open a pull request; our