docs(collections): Add warning about mandatory `_`-prefix (#1091)

* docs(collections): Add warning about mandatory `_`-prefix

* Apply recommended wording

docs/configuration.md

@@ -233,6 +233,9 @@ ga_tracking: "UA-1234567-89,G-1AB234CDE5"
 By default, the navigation and search include normal [pages](https://jekyllrb.com/docs/pages/).
 You can also use [Jekyll collections](https://jekyllrb.com/docs/collections/) which group documents semantically together.
 
+{: .warning }
+> Collection folders always start with an underscore (`_`), e.g. `_tests`. You won't see your collections if you omit the prefix.
+
 For example, put all your test files in the `_tests` folder and create the `tests` collection:
 
 ```yaml