Add instructions

[t4lz]

Let's add a simple .md file with instructions users can follow, to try out various mirrord features.

This tutorial could, for example, at some point tell the users to set a break point in their IDE in the function that handles requests, and then tell them to send a request with curl. The tutorial could then instruct them to add an http filter and send different requests that match and don't match the filter.

I think this tutorial should be mostly about trying mirrord out and getting an idea of what it can do, and not about learning how to use mirrord, so it's ok to just tell the reader exactly what to do, without too much teaching and explaining. What is important in my opinion is that the reader understands what is being achieved, and sees how easy it can be.

[t4lz]

Closing in favour of https://github.com/metalbear-co/mirrord.dev/issues/62