Third party regtest integration test doc, with special attention to protocol upgrades.

[nathan-at-least]

Starting with Overwinter and beyond, we need to make it easy for third party devs to test their code against the new consensus rules in their steady state, and especially around the activation height during rule transitions.

Let's add a (rich) text doc somewhere appropriate in our source tree that describes how to simulate network upgrades between rulesets.

[mms710]

This is a duplicate of this issue: https://github.com/Electric-Coin-Company/ecosystem/issues/61. We will track progress in the other ticket and close this when it is done.

[mms710]

@nathan-at-least have there been users or partners who have asked for this? We're trying to verify if this is still needed after we've added more tests and test documentation and it would be helpful to get the information from a user to understand their use case.