readme for Hackage package servant

[peterbecich]

Currently https://hackage.haskell.org/package/servant has a short description that points to the tutorial.
The intent of this PR is to add more to this page so that it serves as an overview for the entire Servant project. I think this would be helpful because this package servant has the name of the entire project instead of a qualified name i.e. servant-api. https://hackage.haskell.org/package/servant may be the first introduction to servant for some.

Perhaps the same list of packages could be added to https://www.servant.dev/ ?

I don't think the Hackage Tag https://hackage.haskell.org/packages/tag/servant is sufficient for this because it includes packages not part of the official project.

[tchoutri]

@peterbecich Hi, thanks for tackling this. I don't believe the README should contain a list of packages to such an extent. It should say first and foremost:

1. What servant is
2. Point to the tutorials

Remember that this README will then be bundled in the distribution uploaded to Hackage, and cannot be replaced when it gets out of date. I'd rather put down stuff that is hard to bitrot, and have the more volatile kind of documentation live in the Wiki, or in the docs site. :)