Mention #1319 as a pitfall

Author: jy95

docs/common-pitfalls.md

@@ -75,6 +75,10 @@ test(t => {
 });
 ```
 
+### Helpers are not compiled when using a non-default test folder
+
+This is a [known issue](https://github.com/avajs/ava/issues/1319). You should put your tests in a folder called `test` or `__tests__`.
+
 ---
 
 Is your problem not listed here? Submit a pull request or comment on [this issue](https://github.com/avajs/ava/issues/404).