readme: add note about uncaught exceptions

fixes issue avajs#917

Author: arve0

readme.md

@@ -435,7 +435,7 @@ test.failing('demonstrate some bug', t => {
 
 AVA lets you register hooks that are run before and after your tests. This allows you to run setup and/or teardown code.
 
-`test.before()` registers a hook to be run before the first test in your test file. Similarly `test.after()` registers a hook to be run after the last test. Use `test.after.always()` to register a hook that will **always** run once your tests and other hooks complete. `.always()` hooks run regardless of whether there were earlier failures, so they are ideal for cleanup tasks.
+`test.before()` registers a hook to be run before the first test in your test file. Similarly `test.after()` registers a hook to be run after the last test. Use `test.after.always()` to register a hook that will **always** run once your tests and other hooks complete. `.always()` hooks run regardless of whether there were earlier failures, so they are ideal for cleanup tasks. There are two exceptions to this however. If you use `--fail-fast` AVA will stop testing as soon as a failure occurs, and it won't run any hooks including the `.always()` hooks. Uncaught exceptions will crash your tests, possibly preventing `.always()` hooks from running.
 
 `test.beforeEach()` registers a hook to be run before each test in your test file. Similarly `test.afterEach()` a hook to be run after each test. Use `test.afterEach.always()` to register an after hook that is called even if other test hooks, or the test itself, fail. `.always()` hooks are ideal for cleanup tasks.