Readme should include basic installation instructions

[jwilling]

Installation on the Mac is rather straightforward, but on iOS it's a little bit more prone to error due to the additional linker flag needed. A section in the readme would be fantastic so people just getting started won't be frustrated with runtime errors. 🤘

[dmitrym0]

Also should probably mention that All arches should be built, not just active.

[tonyarnold]

@dmitrym0 why is it necessary to build all architectures? The debug configuration generally only builds the current architecture (which saves compile time), but the release configuration will generally always build all the appropriate architectures.

[jspahrsummers]

Combining with #13.