Updated readme include GCF docs

Author: wyattjoh

README.md

@@ -57,6 +57,27 @@ All files from `package/include` will be included in the final build file. See [
 
 ## Usage
 
+### Google Cloud Functions
+
+When using with Google Cloud Functions via the [serverless-google-cloudfunctions](https://github.com/serverless/serverless-google-cloudfunctions)
+plugin, you simply have to provide a `main` field in your `package.json`:
+
+```js
+{
+  // ...
+  "main": "handler.js",
+  // ..
+}
+```
+
+And this plugin will automatically compile your typescript correctly. Note
+that the field must refer to the compiled file name, namely, ending with a `.js`
+extension.
+
+If a `main` field was not found, then this plugin will use `index.js`. Before
+compilation begins, it will check to see that the file indicated exists with a
+`.ts` extension before actually trying to compile it.
+
 ### Automatic compilation
 
 The normal Serverless deploy procedure will automatically compile with Typescript: