Add README badges (#23)

### 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.

README.md

@@ -1,13 +1,17 @@
 # URLSession Transport for Swift OpenAPI Generator
 
+[![](https://img.shields.io/badge/docc-read_documentation-blue)](https://swiftpackageindex.com/apple/swift-openapi-urlsession/documentation)
+[![](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Fapple%2Fswift-openapi-urlsession%2Fbadge%3Ftype%3Dswift-versions)](https://swiftpackageindex.com/apple/swift-openapi-urlsession)
+[![](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Fapple%2Fswift-openapi-urlsession%2Fbadge%3Ftype%3Dplatforms)](https://swiftpackageindex.com/apple/swift-openapi-urlsession)
+
 A client transport that uses the [URLSession](https://developer.apple.com/documentation/foundation/urlsession) type from the [Foundation](https://developer.apple.com/documentation/foundation) framework to perform HTTP operations.
 
 Use the transport with client code generated by [Swift OpenAPI Generator](https://github.com/apple/swift-openapi-generator).
 
 ## Supported platforms and minimum versions
  | macOS | Linux | iOS | tvOS | watchOS |
  | :-: | :-: | :-: | :-: | :-: |
- | ✅ 10.15+  | ✅     | ✅ 13+ | ✅ 13+ | ✅ 6+ |
+ | ✅ 10.15+ | ✅ | ✅ 13+ | ✅ 13+ | ✅ 6+ |
 
 ## Usage