docs(README): add Usage and Installation

* Fix #1

Author: renaudjenny

README.md

@@ -1,3 +1,74 @@
 # SwiftTTSCombine
 
 A very straightforward Combine wrapper around TTS part of AVFoundation/AVSpeechSynthesizer to allow you using Text to Speech with ease.
+
+## Usage
+
+You can instantiate/inject `TTSEngine` object, it has this behavior
+
+* `func speak(string: String)`: call this method when you simply want to use the TTS with a simple String
+  * subscribe to `isSpeakingPublisher` to know when the utterance starts to be heard, and when it's stopped
+  * subscribe to `speakingProgressPublisher` to know the progress, from 0 to 1
+* `var rateRatio: Float`: set the rate to slow down or accelerate the TTS engine
+* `var voice: AVSpeechSynthesisVoice?`: set the voice of the TTS engine, by default, it's the voice for `en-GB`
+
+Example
+
+```swift
+import Combine
+import SwiftTTSCombine
+
+let engine: TTSEngine = SwiftTTSCombine.Engine()
+var cancellables = Set<AnyCancellable>()
+
+engine.speak(string: "Hello World!")
+
+engine.isSpeakingPublisher
+    .sink { isSpeaking in
+        print("TTS is currently \(isSpeaking ? "speaking" : "not speaking")")
+    }
+    .store(in: &cancellables)
+
+engine.speakingProgressPublisher
+    .sink { progress in
+        print("Progress: \(Int(progress * 100))%")
+    }
+    .store(in: &cancellables)
+
+engine.rateRatio = 3/4
+
+engine.speak(string: "Hello World! But slower")
+```
+
+## Installation
+
+### Xcode
+
+You can add SwiftTTSCombine to an Xcode project by adding it as a package dependency.
+
+1. From the **File** menu, select **Swift Packages › Add Package Dependency...**
+2. Enter "https://github.com/renaudjenny/SwiftTTSCombine" into the package repository URL test field
+
+### As package dependency
+
+Edit your `Package.swift` to add this library.
+
+```swift
+let package = Package(
+    ...
+    dependencies: [
+        .package(url: "https://github.com/renaudjenny/SwiftTTSCombine", from: "1.0.0"),
+        ...
+    ],
+    targets: [
+        .target(
+            name: "<Your project name>",
+            dependencies: ["SwiftTTSCombine"]),
+        ...
+    ]
+)
+```
+
+## App using this library
+
+* [📲 Tell Time UK](https://apps.apple.com/gb/app/tell-time-uk/id1496541173): https://github.com/renaudjenny/telltime