Proposal: Adding example on Condition Variables

Would you be open to having a page on Condition Variables, i.e. sync.Cond? It could go right after the mutex page and I believe it'd be a great resource since neither the go tour nor the docs have good examples/motivation on this synchronization primitive.

I'd be happy to contribute with a PR, but I thought I might ask before devoting the time to it in case the feature is considered too advanced for the tutorial.

And thanks for maintaining such a high quality learning resource!