Add README badges (#70)

### Motivation

Surface the Swift version and platform support status from Swift Package
Index.

### Modifications

Added badges, plus a quick link to the docc docs, to the top of the
README.

### Result

Easier to quickly see our support matrix, plus the quick link to docs.

### Test Plan

Previewed locally.

Author: czechboy0

README.md

@@ -1,5 +1,9 @@
 # Swift OpenAPI Generator Runtime
 
+[![](https://img.shields.io/badge/docc-read_documentation-blue)](https://swiftpackageindex.com/apple/swift-openapi-runtime/documentation)
+[![](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Fapple%2Fswift-openapi-runtime%2Fbadge%3Ftype%3Dswift-versions)](https://swiftpackageindex.com/apple/swift-openapi-runtime)
+[![](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Fapple%2Fswift-openapi-runtime%2Fbadge%3Ftype%3Dplatforms)](https://swiftpackageindex.com/apple/swift-openapi-runtime)
+
 This library provides common abstractions and helper functions used by the client and server code generated by [Swift OpenAPI Generator][0].
 
 ## Overview