Update instructions in README.md

[Vectovox]

To facilitate help keeping SDL up-to-date, I think it would be valuable to include primer material in the README to lower the entry requirements. That material would have to cover both how to update the library and validate that it works.

This is high value, in my opinion, as the current (not yet in use by MG) version already fixes a couple of bugs, and SDL2 has had loads of stable bugfix releases in the last 9 months.

Related issues

• Update MonoGame.Library.SDL dependency to 2.28.4.8 MonoGame#8085 (Mentions the validation holding up migrating MonoGame)
• Should we keep loading SDL_GameControllerDB? MonoGame#8408 (The controller DB removal, which will be even more incentive to keep SDL up-to-date)

I'd happily contribute to both, but my unknown unknowns mean I don't know what I may break. That said, I'd happily snag pointers to look into it.