Great idea and you can do this with GitRoyalty! You can hide any files you want behind the subscription–it doesn't have to be the manifest or build script. So in your case you can hide a docs.md file.
Like any file that you would hide with GitRoyalty, you could always share the full repo once you have access to it (probably illegally)
I believe documentation would become outdated sooner than build scripts with new releases, and would require effort to maintain (more than build scripts), which would better justify to pay.
Anyway, the idea of GitRoyalty seems not to make technically impossible to share the payed content, but rather to propose a way for honest people to remunerate honest developers and library maintainers.
