Update "Add or modify a reference catalogue" [Risolto]

The Add or modify a reference catalogue guidelines should be updated to say:

1. If the book isn't listed under Literature create the listing and wait for approval.
2. Create a post to add the reference code with a link to the book, the proposed reference code, some examples of items in Numista covered by the reverence (and maybe other information).

Hello,

Thank you, the guidelines have been updated.