Update README.md (#15)

Author: koddsson

README.md

@@ -2,3 +2,51 @@
 
 A Lit compatible implementation of the [context-protocol community protocol](https://github.com/webcomponents-cg/community-protocols/blob/main/proposals/context.md).
 
+## Installation
+
+```sh
+npm install --save @koddsson/context-protocol
+```
+
+## Usage
+
+A component that implements the ProviderMixin will become a _Provider_ of data and a component that implements the ConsumerMixin will become a _Consumer_ of data.
+
+```ts
+import { ProviderMixin } from "@koddsson/context-protocol";
+
+export class ProviderElement extends ProviderMixin(HTMLElement) {
+  // Set any data contexts here.
+  contexts = {
+    "number-of-unread-messages": () => {
+      return 0;
+    },
+  };
+
+  async connectedCallback() {
+    // It's also possible to provide context at any point using `updateContext`.
+
+    const response = await fetch('/api/messages/');
+    const data = await response.json();
+    this.updateContext('number-of-unread-messages', data.unreadCount);
+  }
+}
+```
+
+```ts
+import { ConsumerMixin } from "@koddsson/context-protocol";
+
+export class ConsumerElement extends ConsumerMixin(HTMLElement) {
+  contexts = {
+    // Fetch contexts that we care about and subscribe to any changes.
+    "number-of-unread-messages": (count: number) => {
+      this.textContent = `${count} unread messages!`;
+    },
+  };
+
+  connectedCallback() {
+    // It's also possible to get any context on demand without subscribing.
+    this.textContent = this.getContext('number-of-unread-messages');
+  }
+}
+```