Add documentation and verification code in package scanner for donation links

[xTrayambak]

• (add) URL validation for URLs in donation field
• (add) documentation on how to add donation links

See this PR and this issue This PR adds documentation as to how one can use the new donation field alongside some verification code in the package scanner.

Due to how Nimble parses package lists, this field will simply be ignored by older versions of Nimble and shouldn't cause any issues. But still,
Do not merge this until the aforementioned PR in Nimble has been merged!