Provide comment for obscure mathing.

[MicahZoltu]

https://github.com/Uniswap/uniswap-v2-core/blob/d2ddbd95e8500cfd3669ab81b64f48956521ea6b/contracts/UniswapV2.sol#L55-L61

This is another place where commentary would be immensely valuable. The math being done here is sufficiently obscure to make it opaque to the reader what is actually happening here and, most importantly, why it is happening. This is obviously not a naive implementation, which is OK if there is a good reason for it to not be a naive implementation, but without any comments it just looks like an engineer being clever, and clever engineers are Bad™.

[NoahZinsmeister]

closed by #45