Add test and documentation for Combine integration

Author: sharplet

README.md

@@ -269,6 +269,25 @@ extension IntOrString: Codable {
 This is described in more details in PR [\#119](https://github.com/MaxDesiatov/XMLCoder/pull/119) 
 by [@jsbean](https://github.com/jsbean) and [@bwetherfield](https://github.com/bwetherfield).
 
+## Integrating with [Combine](https://developer.apple.com/documentation/combine)
+
+When Apple's Combine framework is available, `XMLDecoder` conforms to the
+`TopLevelDecoder` protocol, which allows it to be used with the
+`decode(type:decoder:)` operator:
+
+```swift
+import Combine
+import Foundation
+import XMLCoder
+
+func fetchBook(from url: URL) -> AnyPublisher<Book, Error> {
+    return URLSession.shared.dataTaskPublisher(for: url)
+        .map(\.data)
+        .decode(type: Book.self, decoder: XMLDecoder())
+        .eraseToAnyPublisher()
+}
+```
+
 ## Installation
 
 ### Requirements

Tests/XMLCoderTests/CombineTests.swift

@@ -0,0 +1,36 @@
+//
+//  CombineTests.swift
+//  XMLCoder
+//
+//  Created by Adam Sharp on 9/29/19.
+//
+
+#if canImport(Combine)
+import Combine
+import Foundation
+import XCTest
+import XMLCoder
+
+private let xml = """
+<?xml version="1.0" encoding="UTF-8"?>
+<foo>
+    <name>Foo</name>
+</foo>
+""".data(using: .utf8)!
+
+private struct Foo: Decodable {
+    var name: String
+}
+
+@available(iOS 13.0, macOS 10.15.0, macCatalyst 13.0, tvOS 13.0, watchOS 6.0, *)
+class CombineTests: XCTestCase {
+    func testDecodeFromXMLDecoder() {
+        let data = Just(xml)
+        var foo: Foo?
+        _ = data.decode(type: Foo.self, decoder: XMLDecoder()).sink(receiveCompletion: { _ in }) {
+            foo = $0
+        }
+        XCTAssertEqual(foo?.name, "Foo")
+    }
+}
+#endif