Add supported OS list to README.md?

[liuslevis]

Failed to build on OSX. And I found the answer in a closed issue: macOS is not supported.

Can anyone add supported OS list to README.md?

[tkoeppe]

Sure, we can make that more explicit. I'll earmark it for the next update.

[tkoeppe]

I created a general list of requirements and FAQs in #41. We'll eventually update the documentation proper, but I think keeping a visible list directly in the issues section may be a good idea regardless.