Add a link to the SRComp Docs

[trickeydan]

Addresses #54

[PeterJCLaw]

While this is a potentially useful thing to add, I suggest that what we want a bit more context before we close #54. I suspect that a link to SRComp would be clearer in the context of a page about the competition, rather than as named entry in the sidebar (though maybe both are useful) -- people who don't already know what SRComp is aren't likely to find this link useful.