Docs should include instantiation of MiningChain

[stnbu]

• py-evm Version: 0.4.0-alpha.4

What is wrong?

The documentation mentions MiningChain at the bottom of docs/cookbook/index.rst, however docs/guides/understanding_the_mining_process.rst does not show an example of MiningChain instantiation. A chain_class is assumed to exist from the start of the mining guide.

How can it be fixed

Making the code in docs/guides/understanding_the_mining_process.rst self-contained (so, including a MiningChain instantiation) would help a lot. As a total noob, I have had a lot of trouble getting the mining example to work.