Review and improve wrap creation docs

From https://github.com/mesonbuild/protobuf/issues/9 by @jmansion

> Anyway - can we have some clear guidelines on how to prepare a pull request to fix issues? I'm quite happy about the patch downloads to be zips, but I'd rather the contents were plain and that also the wrap source could be redirected to a local filesystem. It seems a bit obfuscated at the moment, and clearly a wrap contributor seems to be under no obligation to test across many platform and compiler combinations.

Review the existing documentation for creating and contributing wrap files, using the linked mesonbuild/protobuf issue as context for the requested guidance. Clarify pull-request preparation, patch downloads, local filesystem sources, and platform/compiler testing expectations; done means these workflows are explained clearly for wrap contributors.

Written by the indexing model from the issue text.