Leave better instructions on how to update VERSION_LONG

[mokagio]

While working on 1.11.1, I got this error from App Store Connect.

That was due to e9ca832. An easy mistake to make given the comment suggests using a SemVer value instead of a progressive integer 😅 .

Lacking automation to update the values in config/Version.public.xcconfig, we should at least leave a clearer comment on how to set and update VERSION_LONG. 👍

Opening this as an issue to avoid forgetting about it.