Update OAuth docs

docs/getting-started/authentication.md

@@ -7,7 +7,7 @@ You have several options for how you want to connect, but most projects should u
 
 - [Service Account](#service-account) - connects as a specific "bot" user generated by google for your application
 - [API-key](#api-key) - only identifies your application, provides read-only access
-- [OAuth](#oauth) - connect on behalf of a specific user (coming soon!)
+- [OAuth](#oauth) - connect on behalf of a specific user 
 
 **👉 BUT FIRST -- Set up your google project & enable the sheets API 👈**
 1. Go to the [Google Developers Console](https://console.developers.google.com/)
@@ -98,4 +98,32 @@ doc.useApiKey(process.env.GOOGLE_API_KEY);
 ## 👨‍💻 OAuth 2.0 :id=oauth
 **connect on behalf of a user with an Oauth token**
 
-!> Support and instructions coming soon
+Use [Google's OAuth2Client](https://github.com/googleapis/google-auth-library-nodejs#oauth2) to authenticate. 
+
+```javascript
+const = oAuth2Client = new OAuth2Client({
+  clientId: process.env.GOOGLE_OAUTH_CLIENT_ID,
+  clientSecret: process.env.GOOGLE_OAUTH_CLIENT_SECRET
+})
+
+// Pre-configure the client with credentials you have stored in e.g. your databse
+// At a minimum, provide only the refresh_token. The client will use it to retrieve
+// a fresh access_token, with a fresh expiration date.
+oAuth2Client.credentials.access_token = accessToken;
+oAuth2Client.credentials.refresh_token = refreshToken;
+oAuth2Client.credentials.expiry_date = expiryDate; // Unix epoch milliseconds
+
+
+// Listen in whenever a new access token is obtained. You might want to store them in your database.
+// Mind that the refresh_token never changes (unless it's revoked, in which case your end-user will
+// need to go through the full authentication flow again), so storing the new access_token is optional. 
+oAuth2Client.on('tokens', credentials => {
+  console.log(credentials.access_token);
+  console.log(credentials.scope);
+  console.log(credentials.expiry_date);
+  console.log(credentials.token_type); // will always be 'Bearer'
+})
+
+const doc = new GoogleSpreadsheet('<YOUR-DOC-ID>');
+doc.useOAuth2Client(oauth2Client);
+```