Document the Haskell Language Server versioning policy

[pepeiborra]

It's clearly not the PVP, so it ought to be documented somewhere for the sake of future contributors and maintainers.

The documentation should include answers for the following questions:

1. Motivation for deviating from the PVP, and tradeoffs
2. Specification: lower bounds, upper bounds, and version bumps, for all the categories of packages.
   There are 3 categories imho:
   1. core packages
   2. plugin packages
   3. the HLS package
3. Examples of releases that involve interactions with Hackage:
   1. Major release
   2. Bug fix release for core packages
   3. Bug fix release for plugin packages
   4. Bug fix release for HLS

[jneira]

We are following pvp almost strictly in practice, not sure if it still is needed to document it somewhere

[pepeiborra]

That's great news, will close then