Add descriptions for game mechanics

[davcri]

I just added a simple game design document.

Many chapters/mechanics don't have descriptions. We need to fill them: https://github.com/crystal-bit/space-shooter/blob/master/game-design-doc/game-design-document.md

[davcri]

Game design document was updated by PR #17

There is still a lot to improve in the GDD (from now on, abbreviation for Game Design Document).

E.g:

There are different wave movements and formations, randomly selected at the start of each one.

https://github.com/crystal-bit/space-shooter/blob/master/game-design-doc/game-design-document.md#enemy-spawn

How many formations there are? How many type of movements? It would be useful to have some rough sketch for the game programmers.

[Khelthos]

I can work on this, on the pull requests we can discuss about changes in case, ok?

[Khelthos]

Just an update @davcri, I'm checking other same type game mechanics designs to not present misleading information.