Generate documentation in the `duniverse/` folder

[Leonidas-from-XIV]

While discussing the possible ways to use opam-monorepo I raised the point that for someone who is not aware of opam-monorepo the duniverse/ folder with all the sources can be confusing: how was it created, are there forked versions, why is it there, how to update packages from there etc.

To help with the confusion, opam-monorepo could generate a documentation file, e.g. a README.md in the duniverse folder automatically to document what it is and how to use it.

This has a number of advantages

• When committing the duniverse/ folder it serves as documentation, so even if the project has not documented its usage of opam-monorepo there will be some generic information in place. It could potentially also mention the versions of the packages and links to them.
• Github would display it inline when opening the folder in the browser.
• When not committing it it doesn't hurt, as it is just a small-ish text file.