Add example of using yarn test with arguments (#1383)

Not sure if the copy is great, but since this is mostly for n00bs, better to err on side of more information.

README.md

@@ -473,9 +473,9 @@ yarn test
 Here's what you need to know about the tests:
 
 * The tests uses [Jest](https://facebook.github.io/jest/) snapshots.
-* You can make changes and run `jest -u` to update the snapshots. Then run `git
-  diff` to take a look at what changed. Always update the snapshots when opening
-  a PR.
+* You can make changes and run `jest -u` (or `yarn test -- -u`) to update the
+  snapshots. Then run `git diff` to take a look at what changed. Always update
+  the snapshots when opening a PR.
 * You can run `AST_COMPARE=1 jest` for a more robust test run. That formats each
   file, re-parses it, and compares the new AST with the original one and makes
   sure they are semantically equivalent.