fix: Relates to #487. Update readme with instructions to add missing dependencies

[ltfschoen]

No description provided.

[ozkriff]

Thanks for the PR. I agree that it's a good idea to add some information about the required system deps to the README, but adding detailed docs for each platform seems superfluous to me. It's better to just link to the appropriate engine's docs - https://github.com/ggez/ggez/blob/a62acaa24/docs/BuildingForEveryPlatform.md

(I'm going to make the abovementioned change in a few hours)