Not sarcastic at all. First, I said README.md. Markdown in readmes was popularized - if not invented by - GitHub. Furthermore, the top billing of README.md files in the GitHub UI took over from more traditional meta-level documentation that used to be in separate places, such as an "About" field/page on SourceForge or a separate web page. I much prefer this documentation in repository. Even if projects had READMEs, they might not include certain information there previously.

All these things are just GitHub's implementation and UX details. The README file is the traditional meta-level documentation (if I correctly understand what you mean by that: top level document that gives a more or less comprehensive overview of the project). READMEs always included this information, sometimes offloading details of particular topics into INSTALL, AUTHORS, LICENCE, etc. files. GitHub just figured they might as well use the thing as intended and actually render it on a web page. As for markup, people were already using email like markup in those files, so this is again just following established practice to its logical conclusion and modern implementations. GH's sole innovation in convention here is tacking on the very annoying file type extension in order to not have to figure out the format automatically.