Mention CSL validator in the README

[PgBiel]

We should mention some more information on CSL, including how to troubleshoot common style problems, but also add this link https://validator.citationstyles.org, which shows what can go wrong in a style For example, in this style we can see some otherwise unknown variables being used, leading to an otherwise obscure error like "data did not match any variant of untagged enum Variable".