added auth docs and images for GitHub and Twitter

docs/guides/auth.md

@@ -0,0 +1,77 @@
+# Guide - Authentication
+
+### Twitter
+1. Sign-in or sign-up for a Twitter account
+2. Go to the Twitter Application management page [here](https://apps.twitter.com/)
+3. Click on the **Create New App** button to create a new Twitter app:
+
+![create-twitter-app](images/auth/create-twitter-app.png)
+
+4. Fill out the create application form, check the developer agreement box, and click **Create Your Twitter Application**
+
+![register-twitter-application](images/auth/register-twitter-application.png)
+
+*Note: you may have to register your phone number with Twitter to create a Twitter application*
+
+To do this Click your profile icon --> Settings and privacy --> Mobile  --> Select Country/region --> Enter phone number --> Click Continue
+
+5. After you receive confirmation that the Twitter application was created, click **Keys and Access Tokens**
+
+![twitter-app-confirmation](images/auth/twitter-app-confirmation.png)
+
+6. Obtain your Twitter Consumer Key and Consumer Secret
+
+![twitter-app-keys](images/auth/twitter-app-keys.png)
+
+7.  Add your Consumer Key and Consumer Secret to your config.json file or pass them as environment variables:
+    * config.json:
+      ````javascript
+      {
+        "production": {
+          "twitter": {
+              "consumerKey": "esTCJFXXXXXXXXXXXXXXXXXXX",
+              "consumerSecret": "zpCs4tU86pRVXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"
+          }
+        }
+      }
+      ````
+    * environment variables:
+      ````
+      HMD_TWITTER_CONSUMERKEY=esTCJFXXXXXXXXXXXXXXXXXXX
+      HMD_TWITTER_CONSUMERSECRET=zpCs4tU86pRVXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+      ````
+
+### GitHub
+1. Sign-in or sign-up for a GitHub account
+2. Navigate to developer settings in your GitHub account [here](https://github.com/settings/developers) and select the "OAuth Apps" tab
+3. Click on the **New OAuth App** button, to create a new OAuth App: 
+
+![create-oauth-app](images/auth/create-oauth-app.png)
+
+4. Fill out the new OAuth application registration form, and click **Register Application**
+
+![register-oauth-application-form](images/auth/register-oauth-application-form.png)
+
+*Note: The callback URL is <your-hackmd-url>/auth/github/callback*
+
+5. After successfully registering the application, you'll receive the Client ID and Client Secret for the application
+
+![application-page](images/auth/application-page.png)
+
+6. Add the Client ID and Client Secret to your config.json file or pass them as environment variables
+    * config.json:
+      ````javascript
+      {
+        "production": {
+          "github": {
+              "clientID": "3747d30eaccXXXXXXXXX",
+              "clientSecret": "2a8e682948eee0c580XXXXXXXXXXXXXXXXXXXXXX"
+          }
+        }
+      }
+      ````
+    * environment variables:
+      ````
+      HMD_GITHUB_CLIENTID=3747d30eaccXXXXXXXXX
+      HMD_GITHUB_CLIENTSECRET=2a8e682948eee0c580XXXXXXXXXXXXXXXXXXXXXX
+      ````