Add "installation" section to README

The first time I opened the Wireframe kit, I got warnings from Sketch that I was missing fonts that it uses. I thought that the README might benefit from a short "installation" section that mentions requirements and suggested fonts. Let me know if I missed any fonts you used that I may have already had installed and I can edit the PR. Thanks for the great kit!
Merge remote-tracking branch 'davidcelis/patch-1'