Update README (#4)

Put some stuff about usage in the "Usage" section

README.md

@@ -13,6 +13,47 @@ Encryption](https://en.wikipedia.org/wiki/End-to-end_encryption)) for
 
 ## Usage
 
+1. Install in your project:
+
+    ```
+    npm install --save @matrix-org/matrix-sdk-crypto-wasm
+    ```
+
+    or:
+
+    ```
+    yarn add @matrix-org/matrix-sdk-crypto-wasm
+    ```
+
+2. Import the library into your project and initialise it.
+
+    It is recommended that you use a dynamic import, particularly in a Web
+    environment, because the WASM artifiact is large:
+
+    ```javascript
+    async function loadCrypto(userId, deviceId) {
+        const matrixSdkCrypto = await import("@matrix-org/matrix-sdk-crypto-wasm");
+        await matrixSdkCrypto.initAsync();
+
+        // Optional: enable tracing in the rust-sdk
+        new matrixSdkCrypto.Tracing(matrixSdkCrypto.LoggerLevel.Trace).turnOn();
+
+        // Create a new OlmMachine
+        //
+        // The following will use an in-memory store. It is recommended to use
+        // indexedDB where that is available.
+        // See https://matrix-org.github.io/matrix-rust-sdk-crypto-wasm/classes/OlmMachine.html#initialize
+        const olmMachine = await RustSdkCryptoJs.OlmMachine.initialize(
+            new RustSdkCryptoJs.UserId(userId),
+            new RustSdkCryptoJs.DeviceId(deviceId),
+        );
+
+        return olmMachine;
+    }
+    ```
+
+## Building
+
 These WebAssembly bindings are written in [Rust]. To build them, you
 need to install the Rust compiler, see [the Install Rust
 Page](https://www.rust-lang.org/tools/install). Then, the workflow is
@@ -25,15 +66,11 @@ following commands:
 
 ```sh
 $ yarn install
-$ yarn build
+$ yarn build  # or 'yarn build:dev' to make an unoptimised build
 $ yarn test
 ```
 
-A `matrix_sdk_crypto.js`, `matrix_sdk_crypto.d.ts` and a
-`matrix_sdk_crypto_bg.wasm` files should be generated in the `pkg/`
-directory.
-
-TBD
+The compiled output should be generated in the `pkg/` directory.
 
 ## Documentation