Added a brief comment explanation of how passport strategy works

config/passport.js

@@ -38,6 +38,21 @@ passport.use(new LocalStrategy({ usernameField: 'email' }, function(email, passw
   });
 }));
 
+/**
+ * OAuth Strategy Overview
+ *
+ * - User is already logged in.
+ *   - Check if there is an existing account with a provider id or email.
+ *     - If there is, return an error message. (Account merging not supported)
+ *     - Else link new OAuth account with currently logged-in user.
+ * - User is not logged in.
+ *   - Check if it's a returning user.
+ *     - If returning user, sign in and we are done.
+ *     - Else check if there is an existing account with user's email.
+ *       - If there is, return an error message.
+ *       - Else create a new account.
+ */
+
 /**
  * Sign in with Facebook.
  */