Update CONTRIBUTING.md
Nightly commit

- Added variable grid size
- Fixed 2048 link
- Added game modes
- Added a partial menu screen
Menu Changes

- Rearanged menu and try again screens
- Reorganised timers
Nightly changes

- Began to add multidifficulty best scores
- Reset difficulties each game
- Added button selection
Fixed best scores

Fixed best scores to give best score for each size/diff
Fixed winning score that was changed for test purposes
Merge pull request #1 from tommykent1210/nightly

Monday 31st March Pull form Nightly