Update signals runtime readme (#1249)

Author: silesky

packages/signals/signals-runtime/README.md

@@ -1,10 +1,69 @@
 # @segment/analytics-signals-runtime
 Encapsulates Signals runtime functionality, in order to share logic between the signals plugins for browser and mobile.
 
+### Installation
+```bash
+# npm
+npm install @segment/analytics-signals-runtime
+# yarn
+yarn add @segment/analytics-signals-runtime
+# pnpm
+pnpm install @segment/analytics-signals-runtime
+```
+
+### Usage
+
+### Importing the signals runtime as a module (e.g. `signals.find()`)
+
+```ts
+import { WebSignalsRuntime, Signal } from  '@segment/analytics-signals-runtime'
+// MobileSignalsRuntime
+// WebRuntimeConstants,
+// MobileRuntimeConstants,
+
+const mockSignalBuffer: Signal[] = [{
+  type: "network",
+  data: {
+    action: "response",
+    url: "https://segment-integrations.github.io/segment-shop-auto/cart",
+    data: { foo: "bar" },
+    ok: true,
+    status: 201,
+    contentType: "application/json",
+    page: {
+      path: "/segment-shop-auto/",
+      referrer: "",
+      title: "Segment Shop - Home Collection - Timecraft",
+      search: "",
+      url: "https://segment-integrations.github.io/segment-shop-auto/#/home-collection/Timecraft",
+      hostname: "segment-integrations.github.io",
+      hash: "#/home-collection/Timecraft"
+    }
+  }
+}]
+
+const signals = new WebSignalsRuntime(mockSignalBuffer)
+
+const networkSignal = signals.find(null, 'network')
+
+```
+### Importing the signals runtime a JS string
+```ts
+import { getRuntimeCode, getMobileRuntimeCode } from '@segment/analytics-signals-runtime'
+
+eval(`
+ ${getMobileRuntimeCode()}
+ signals.signalBuffer = [....]
+ `)
+```
+
+### API documentation
+For all exports, explore [the index.ts](src/index.ts).
+
 ### Development
 `yarn build` generate the following artifacts:
 | Generated File(s) | Description | 
 |--------|-------------|
 | `/dist/runtime/index.[platform].js`, `/[platform]/get-runtime-code.generated.js` | Exposes `globalThis.Signals` and constants (e.g. `SignalType`), either through the script tag or in the mobile JS engine | 
 | `/dist/editor/[platform]-editor.d.ts.txt` | Type definitions for monaco editor on app.segment.com 
-| `/dist/esm/index.js` | Entry point for `@segment/analytics-signals` and the segment app, for consumers that want to consume the types / runtime code as an npm package. | 
+| `/dist/esm/index.js` | Entry point for `@segment/analytics-signals` and the segment app, for consumers that want to consume the types / runtime code as an npm package. |