Support READMEs for packages inside monorepos

[dmfrancisco]

We currently get the READMEs already rendered via GitHub API. This allows us to support GitHub Task Lists, allows us to be sure that the README always looks as expected, and also support READMEs that are not written in Markdown. But for monorepos, where there are several READMEs per repository (one for each package), it would be preferable to use the README NPM provides (or alternatively, somehow find those files inside the repo and still use the GitHub API).

Currently, the main README is shown for all subpackages, which is not helpful, as seen here.