The coins can be earned through the donation of resources to the alliance treasury. You get a coin after each set amount of donated resources:

So if you donated 2000 Copper and 1000 Silver, you have four Alliance Coins in total. But if you donated 2200 Copper, you still got only two. Only when you reach an donated amount of 3000 Copper, you earn the third Alliance Coin.

 

If you leave an alliance and rejoin later, your earned Alliance Coins will still be present.

 

The Alliance Coins allow to see how much which alliance member has donated. At the same time, it is a new member-ranking, in which the player with the most coins is shown on top of the list.

 

More features for the Alliance Coins are already planned!