Add some text about locking and multithreading in the game

Endless sky appears to use multi-threading and locking.

If developers inexperienced with the codebase (or inexperienced with threading) start making code-changes to structures shared between threads, then the risk of race-conditions and deadlocks could increase.

This request is to get some text in this documentation that describes which threads are used in the game and some text on which classes/areas/datastructures are expected to use locking (and when).