Add 'tips'/'hints'

[hhaslam11]

Not sure how exactly to implement this. Some options I can think of:

• Randomly appears
• Needs to unlock in store using either XP or coins
• New tip on level-up
• New tip whenever you kill an enemy
• New tip every x turns

Tips would just be useful little messages that the player might not think of. For example,

When you encounter a new enemy, look up the max damage it can do in the help section.
Then you can see how low your health can safely get before you should revive.