Update README.md

Author: mrousavy

README.md

@@ -20,7 +20,60 @@ Nitro Modules are highly efficient native modules exposed with a statically comp
 
 ## Usage
 
-🤔
+### 1. Create a TypeScript spec
+
+The TypeScript spec is the single source of truth. It's interfaces, enums or other type declarations will be converted to C++ (or Swift/Kotlin) types using a code generator.
+
+```ts
+export interface MathsModule {
+  readonly pi: number
+  multiply: (a: number, b: number) => number
+  subtract: (a: number, b: number) => number
+  add: (a: number, b: number) => number
+}
+```
+
+### 2. Implement the native interface
+
+In C++:
+
+```cpp
+class MathsModule: public MathsModuleSpec {
+public:
+  double getPi() override { return PI; }
+
+  double multiply(double a, double b) override { return a * b; }
+  double subtract(double a, double b) override { return a - b; }
+  double add(double a, double b) override { return a + b; }
+};
+```
+
+In Swift:
+
+```swift
+class MathsModule: MathsModuleSpec {
+  var Pi: Double {
+    return .pi
+  }
+
+  func multiply(a: Double, b: Double) -> Double { return a * b }
+  func subtract(a: Double, b: Double) -> Double { return a - b }
+  func add(a: Double, b: Double) -> Double { return a + b }
+}
+```
+
+In Kotlin:
+
+```kotlin
+class MathsModule: MathsModuleSpec {
+  val Pi: Double
+    get() = PI
+
+  fun multiply(a: Double, b: Double): Double { return a * b }
+  fun subtract(a: Double, b: Double): Double { return a - b }
+  fun add(a: Double, b: Double): Double { return a + b }
+}
+```
 
 ## Platforms