Mention updating appcast’s sha256 in contributing.md

[vitorgalvao]

I’m currently updating a bunch of casks to their latest versions. In some cases, that means updating simply the appcast’s :sha256 (as everything else is fine, though I am checking every one of them individually).

Some of them have versionless URLs, and some others (so far, just one), were switched from a versioned to a versionless URL (since we can take advantage of CDN like that). Since those casks have appcasts, that is actually a good thing since we can still keep the advantages of versions.

After updating all of them, I’ll start working on the script to check appcasts (that will open issues automatically).

@caskroom/maintainers This means that (finally) from now on updates to casks must include updates to appcast’s :sha256 as well, and we must ask contributors do so/fix the PRs when they miss it. This also means there should be a short description in CONTRIBUTING.md to explain why and how to do it.

[adidalal]

I can take care of it as part of #16373

Will be working on that over the next day or so.

[vitorgalvao]

Thank you for doing so. Closing in favour of #16373.