Add transit tour

[troglotit]

I've heard about Transit for quite a long time, but haven't really understood what it is/how it worked. Yesterday I watched David Nolen's talk on Transit at LispNYC, and only there I found https://cognitect.github.io/transit-tour/, and I found to quite easy to grok, compared to README, which is written in official tone (of course it has to be a full reference).

Maybe we should add transit-tour in README/wiki? It would be a goto-reference to share with a colleague, which would help with adopting Transit.