Clarify direnv setup instructions

[mkoval]

I missed the task workstation:direnv command in the README because of the way it is formatted. Instead of being called out in a code block, like the other steps, it is inlined as part of another step. It is also not obvious how to tell whether the setup was successful.

This sent me down a bit of a rabbit hole because several of the following steps fail in surprising ways (e.g. TLS errors) when the env variables are missing. I thought I had setup direnv correctly, but actually had missed a step.

This PR modifies the README to callout task workstation:direnv as a separate step and explains how to check whether it was successful. Hopefully this will will prevent someone else from going down the same rabbit hole. 😄

P.S. direnv is really neat! I am going to start using it everywhere!

[onedr0p]

Thanks!