docs: added PubSub Typescript doc (#462)

Co-authored-by: Grant Timmerman <[email protected]>

Author: diogosantos

docs/typescript-pubsub.md

@@ -0,0 +1,70 @@
+# PubSub Event Function in Typescript
+
+This guide is an example of how to develop a function to receive PubSub messages in Typescript with the Functions Framework.
+
+1. Use [gts](https://github.com/google/gts) to configure Typescript.
+
+    ```sh
+    npx gts init
+    ```
+
+2. Install the required packages:
+
+    ```sh
+    npm install @google-cloud/functions-framework
+    npm install @google-cloud/pubsub 
+    npm install concurrently nodemon --save-dev
+    ```
+
+3. Replace the contents of `src/index.ts` with:
+
+    ```ts
+    import {Context, EventFunction} from "@google-cloud/functions-framework/build/src/functions";
+    import {PubsubMessage} from "@google-cloud/pubsub/build/src/publisher";
+
+    export const helloPubSub: EventFunction = (message: PubsubMessage, context: Context) => {
+      const data = message.data ? Buffer.from(message.data as string, "base64").toString() : "No Message";
+      console.log(data);
+    };
+    ```
+
+4. Compile your Typescript code:
+
+   ```sh
+   npm run compile
+   ```
+
+   This should compile your `index.ts` to `build/src/index.js`.
+
+## Deploying with gcloud CLI
+
+1. Adjust `main` field in `package.json` to point to the compiled javascript source.
+
+    ```js
+      "main": "build/src/index.js",
+      ...
+    ```
+
+2. Remove `prepare` script in `package.json` created by [gts](https://github.com/google/gts). This is because the `prepare` script requires typescript to be installed and will
+   cause the function to fail to deploy if not removed.
+
+3. Create a Pub/Sub topic, where `COOL_TOPIC` is the name of the new topic you are creating:
+
+   ```sh
+   gcloud pubsub topics create COOL_TOPIC
+   ```
+
+4. Deploy:
+
+    ```sh
+    gcloud functions deploy helloPubSub \
+    --runtime nodejs16 \
+    --trigger-topic COOL_TOPIC
+    ```
+
+5. Publish a message to the topic to test your function:
+
+   ```sh
+   gcloud pubsub topics publish COOL_TOPIC --message="hello"
+   ```
+