Update testing instructions and link to them on the website

[Sjors]

I might make a PR based on the comments in #3 once I've figured it out myself.

[CryptAxe]

There are instructions: https://github.com/drivechain-project/testing but they are very outdated. We are still in the middle of changes to BMM so I wouldn't try doing this right now, but if you do make an updated guide make a PR for https://github.com/drivechain-project/testing.

We should link to the instructions somewhere on the website though @psztorc.

[Sjors]

Thanks, I hadn't found that, so a more prominent link will probably help. I can make a PR against that, with some partial BMM stuff that you can complete the text for later later.