Formalize strategy for upgrading minimum versions of dependencies

We have some statements regarding supported distro versions, but we have no clear statement on how we select and upgrade our minimum required versions.

This is essentially a follow-up of https://github.com/precice/precice.github.io/issues/16.

We need a statement on:
* What dictates our minimum required versions.
* That minimum required version updates are not considered breaking releases.
* What upper bound we actively test.
* How forward-compatible the project is and if changes warrant a bugfix release.
* What distros we directly support (2 LTS of Ubuntu)
* How we deal with different MPI implementations (OpenMPI vs MPICH on Fedora, what about Intel MPI / Windows MPI?)

Optionally:
* What distros we indirectly support if they meet the version requirements (Debian, Arch, Fedora)
* How we handle required bug fixes for indirectly supported distros

Where shall we put this in the docs? Under fundamentals? How shall we call this page?