Tweak update v0.2

After the OPR jam ended, I ended up doing more playtests and making several changes to the ruleset. The original was intended to be challenging, but it ended up being unpleasant, so I made adjustments to the rules such as:

- Removed the enemy crossing cover rule, as I feel like it adds unnecessary complexity without giving much value
- Changed the shooting distance rule to +1 for under 6", so it would be a tad easier but still forces you to make choices
- Added purpose of cover that's easier to understand and more useful to players: units within 1" in cover now gets +1 to shooting
- Added narrative variants for good measure - this is not just a zombie game, you know.
- and others which I kinda forgot

Hope this one's more enjoyable than the last one, and thanks for everyone trying this out. Cheers!