Add documentation for games 2 and 3

[K-Meech]

For #11 and #16

Adds short docs .md files for game 2 and 3. These highlight the most important values to edit during playtesting, and which objects they are associated with.

[thompson318]

This looks good to me and gives a succinct summary of what to change.

I'm wondering whether the other serialised variables in the scripts should not be serialised to avoid confusion, as I guess the intent is that they won't get modified in play testing, but I'm probably over thinking it.

[K-Meech]

Thanks @thompson318 ! I think most of the other serialised fields are values that still need to be set in the editor (+ may be updated by us developers), but aren't likely to be changed during play testing e.g. assigning which star prefab object the star generator should create.
Happy to edit any you think are unnecessary though.