doc(crypto-nodejs): Improve the README.md.

crates/matrix-sdk-crypto-nodejs/README.md

@@ -1,8 +1,30 @@
 # `matrix-sdk-crypto-nodejs`
 
-This is the NodeJS bindings to [`matrix-sdk-crypto`].
+Welcome to the [Node.js] binding for the Rust [`matrix-sdk-crypto`]
+library! This binding is part of the [`matrix-rust-sdk`] project,
+which is a library implementation of a [Matrix] client-server.
 
-## Installation
+`matrix-sdk-crypto-nodejs` is a no-network-IO implementation of a
+state machine, named `OlmMachine`, that handles E2EE ([End-to-End
+Encryption](https://en.wikipedia.org/wiki/End-to-end_encryption)) for
+[Matrix] clients.
+
+## Usage
+
+This Node.js binding is written in [Rust]. To build this binding, you
+need to install the Rust compiler, see [the Install Rust
+Page](https://www.rust-lang.org/tools/install). Then, the workflow is
+pretty classical by using [npm], see [the Downloading and installing
+Node.js and npm
+Page](https://docs.npmjs.com/downloading-and-installing-node-js-and-npm).
+
+The binding is using NAPI 6 (Node API version 6), which means that is
+compatible with Node.js versions 10.20.0, 12.17.0, 14.0.0, 15.0.0 and
+16.0.0 (see [the full Node API version
+matrix](https://nodejs.org/api/n-api.html#node-api-version-matrix)).
+
+Once the Rust compiler, Node.js and npm are installed, you can run the
+following commands:
 
 ```sh
 $ npm install
@@ -10,12 +32,76 @@ $ npm run build
 $ npm run test
 ```
 
-### With tracing
+An `index.js`, `index.d.ts` and a `*.node` files should be
+generated. At the same level of those files, you can edit a file and
+try this:
 
-If you want to get [tracing](https://tracing.rs) (to get logs):
+```javascript
+const { OlmMachine } = require('./index.js');
+
+// Let's see what we can do.
+```
+
+The `OlmMachine` state machine works in a push/pull manner:
+
+* You push state changes and events retrieved from a Matrix homeserver
+  `/sync` response, into the state machine,
+  
+* You pull requests that you will need to send back to the homeserver
+  out of the state machine.
+  
+```javascript
+const { OlmMachine, UserId, DeviceId, RoomId, DeviceLists } = require('./index.js');
+
+async function main() {
+    // Define a user ID.
+    const alice = new UserId('@alice:example.org');
+
+    // Define a device ID.
+    const device = new DeviceId('DEVICEID');
+
+    // Let's create the `OlmMachine` state machine.
+    const machine = await OlmMachine.initialize(alice, device);
+
+    // Let's pretend we have received changes and events from a
+    // `/sync` endpoint of a Matrix homeserver, …
+    const toDeviceEvents = "{}"; // JSON-encoded
+    const changedDevices = new DeviceLists();
+    const oneTimeKeyCounts = {};
+    const unusedFallbackKeys = [];
+
+    // … and push them into the state machine.
+    const decryptedToDevice = await machine.receiveSyncChanges(
+        toDeviceEvents,
+        changedDevices,
+        oneTimeKeyCounts,
+        unusedFallbackKeys,
+    );
+
+    // Now, let's pull requests that we need to send out to the Matrix
+    // homeserver.
+    const outgoingRequests = await machine.outgoingRequests();
+
+    // To complete the workflow, send the requests here out and call
+    // `machine.markRequestAsSent`.
+}
+
+main();
+```
+
+### With tracing (experimental)
+
+If you want to enable [tracing](https://tracing.rs), i.e. to get the
+logs, you should re-compile the extension with the `tracing` feature
+turned on:
 
 ```sh
 $ npm run build -- --features tracing
+```
+
+Now, you can use the `RUST_LOG` environment variable to tweak the log filtering, such as:
+
+```sh
 $ RUST_LOG=debug npm run test
 ```
 
@@ -23,7 +109,13 @@ See
 [`tracing-subscriber`](https://tracing.rs/tracing_subscriber/index.html)
 to learn more about the `RUST_LOG` environment variable.
 
+## Documentation
 
+TBD.
 
-
+[Node.js]: https://nodejs.org/
 [`matrix-sdk-crypto`]: https://github.com/matrix-org/matrix-rust-sdk/tree/main/crates/matrix-sdk-crypto
+[`matrix-rust-sdk`]: https://github.com/matrix-org/matrix-rust-sdk
+[Matrix]: https://matrix.org/
+[Rust]: https://www.rust-lang.org/
+[npm]: https://www.npmjs.com/