Add README badges (#24)

### 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.

Also, expanded the supported platforms, we do actually support iOS and
friends (was an oversight not to include it before).

### Result

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

### Test Plan

Previewed locally.

Author: czechboy0

Diff for: Package.swift

@@ -29,7 +29,7 @@ swiftSettings.append(
 let package = Package(
     name: "swift-openapi-async-http-client",
     platforms: [
-        .macOS(.v10_15),
+        .macOS(.v10_15), .iOS(.v13), .tvOS(.v13), .watchOS(.v6),
     ],
     products: [
         .library(

Diff for: README.md

@@ -1,13 +1,17 @@
 # AsyncHTTPClient Transport for Swift OpenAPI Generator
 
-A client transport that uses the [HTTPClient](https://swift-server.github.io/async-http-client/docs/current/AsyncHTTPClient/Classes/HTTPClient) type from the [AsyncHTTPClient](https://github.com/swift-server/async-http-client) library to perform HTTP operations.
+[![](https://img.shields.io/badge/docc-read_documentation-blue)](https://swiftpackageindex.com/swift-server/swift-openapi-async-http-client/documentation)
+[![](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Fswift-server%2Fswift-openapi-async-http-client%2Fbadge%3Ftype%3Dswift-versions)](https://swiftpackageindex.com/swift-server/swift-openapi-async-http-client)
+[![](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Fswift-server%2Fswift-openapi-async-http-client%2Fbadge%3Ftype%3Dplatforms)](https://swiftpackageindex.com/swift-server/swift-openapi-async-http-client)
+
+A client transport that uses the [HTTPClient](https://swiftpackageindex.com/swift-server/async-http-client/documentation/asynchttpclient/httpclient) type from the [AsyncHTTPClient](https://github.com/swift-server/async-http-client) library 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 |
- | :-: | :-: |
- | ✅ 10.15+ | ✅ |
+| macOS | Linux | iOS | tvOS | watchOS |
+| :-: | :-: | :-: | :-: | :-: |
+| ✅ 10.15+ | ✅ | ✅ 13+ | ✅ 13+ | ✅ 6+ |
 
 ## Usage
 

Diff for: Sources/OpenAPIAsyncHTTPClient/Documentation.docc/Documentation.md

@@ -4,14 +4,14 @@ Send HTTP requests to the server using the AsyncHTTPClient library.
 
 ## Overview
 
-A client transport that uses the [HTTPClient](https://swift-server.github.io/async-http-client/docs/current/AsyncHTTPClient/Classes/HTTPClient) type from the [AsyncHTTPClient](https://github.com/swift-server/async-http-client) library to perform HTTP operations.
+A client transport that uses the [HTTPClient](https://swiftpackageindex.com/swift-server/async-http-client/documentation/asynchttpclient/httpclient) type from the [AsyncHTTPClient](https://github.com/swift-server/async-http-client) library 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 |
-| :-: | :-: |
-| ✅ 10.15+ | ✅ |
+| macOS | Linux | iOS | tvOS | watchOS |
+| :-: | :-: | :-: | :-: | :-: |
+| ✅ 10.15+ | ✅ | ✅ 13+ | ✅ 13+ | ✅ 6+ |
 
 ### Usage