Add installation and usage to readme

README.md

@@ -1,3 +1,112 @@
 # WhoopDI
 
 WhoopDI is a simple dependency injection package for Swift.
+
+# Installation
+
+WhoopDI is available through [Swift Package Manager](https://swift.org/package-manager/). 
+
+To install it, simply add the following line to your `Package.swift` file:
+
+```swift
+dependencies: [
+    .package(url: "https://github.com/WhoopInc/WhoopDI.git", from: "0.0.3.9")
+]
+```
+
+Then include it in your target's dependencies:
+
+```swift
+targets: [
+    .target(
+        name: "YourTarget",
+        dependencies: ["WhoopDIKit"]),
+]
+```
+
+# Usage
+
+There are a few simple steps to setup WhoopDI in your project:
+1. Define dependency modules which defined your dependencies.
+2. Register your dependencies with WhoopDI.
+3. Inject your top level dependencies.
+
+## Dependency Modules
+
+You can define your dependencies in a module by creating a class that conforms to `DependencyModule`. This class should define a `registerDependencies` method which will be called by WhoopDI to register your dependencies.
+
+```swift
+final class MyModule: DependencyModule {
+    override func defineDependencies() {
+        factory { MyConcreteObject() }
+        factory { try MyObject(dependency: self.get()) as MyProtocol }
+        factory(named: "MyNamedObject") { MyNamedObject() as MyNamedProtocol }
+        singleton { try MySingletonObject(dependency: self.get()) }
+    }
+}
+```
+
+A few interesting things to note in the above snippet:
+- `factory` is used to register a factory dependency. This means that every time you request this dependency from WhoopDI, a new instance will be created.
+- `singleton` is used to register a singleton dependency. This means that only one instance of this dependency will be created and returned every time it is requested.
+- `named` is used to register a dependency with a specific name. This is useful when you have multiple dependencies of the same type.
+- `get` is used to retrieve a dependency from the container. This is useful when you need to pass a dependency to another dependency. Note this can throw in testing mode if the dependency is not registered, so a `try` is required.
+
+## Register Dependencies
+
+To register modules with WhoopDI, you can use the `registerModules` method of WhoopDI. This method takes a list of modules to register:
+
+```swift
+WhoopDI.registerModules([MyModule()])
+```
+
+## Inject Dependencies
+
+The simplest way to inject a dependency is to use the `inject` method of WhoopDI.
+
+```swift
+let myFeature: MyFeature = WhoopDI.inject()
+let myNamedFeature: MyNamedFeature = WhoopDI.inject("named")
+```
+
+These inject methods will piece together the dependencies you have registered and return the top level dependency you are requesting. A few notes here:
+- Type lookup happens via generics, so WhoopDI must be able to to infer the type you are requesting.
+- If you have multiple dependencies of the same type, you can use the `named` parameter to specify which dependency you want.
+- If the dependency you are requesting is not registered, WhoopDI will fill throw a fatal error (at runtime).
+- You should inject only your top level dependencies (e.g. view controllers, services, etc.). Most of your code should be unaware of WhoopDI.
+    - In other words, you should not invoke `WhoopDI.inject` in your lower level dependencies. They should be provided in a module, then injected into your top level dependencies.
+
+### Local Module Inject
+
+Sometimes you have local variables which you want to pass into the dependency graph. This can be achieved by using the `inject` with closure method of WhoopDI.`
+
+```swift
+let theAnswer = 42
+WhoopDI.inject { module in
+    module.factory(name: "answer") { theAnswer }
+}
+```
+
+The above introduces the local variable into the dependency graph. It can then be a dependency in an existing module, or you can provide additional factory definitions which depend upon it in the closure.
+
+# Testing
+
+WhoopDI is designed to be easy to test. You can leverage the `WhoopDIValidator` to validate your modules and ensure that all needed dependencies are provided to the dependency graph.
+
+```swift
+class MyDITests: XCTestCase {
+    override func tearDown() async throws {
+        await WhoopDI.removeAllDependencies()
+    }
+
+
+    func test_allDependenciesProvided() {
+        WhoopDI.registerModules(modules: [MyModule()])
+
+        let validator = WhoopDIValidator()
+        validator.validate { error in
+            XCTFail("DI failed with error: \(error)")
+        }
+    }
+}
+```