Added docs for authentication

docs/authentication.md

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+# Authentication
+
+## Devise
+
+[Devise](https://github.com/heartcombo/devise) is a flexible authentication solution that provides a full-featured authentication system that provides a complete MVC solution based on Rails engines. Some of the modules it includes:
+
+**Database Authenticatable**, **Registerable**, **Confirmable**, **Timeoutable**, **Omniauthable**..etc
+
+Here is an example of how Devise is used in our User model:
+
+```
+class User < ApplicationRecord
+  devise :database_authenticatable, :registerable, :confirmable, :trackable, :omniauthable
+end
+```
+
+Devise is customizable, allowing you to add or remove modules that best suit your application. Devise provides many helper methods like `user_signed_in?` that checks if a user is signed in or `current_user` that returns the **User** record of the signed in user.
+
+### Configuration
+
+In the provided codebase, devise in configured in `config/initializers/devise.rb`.
+
+1. If using the Rails mailer, don't forget to set the value of the constant `DEFAULT_FROM_EMAIL` for your application.
+
+2. If modifying anything related to the sign up or confirmation process, refer to the controllers `RegistrationsController` and `ConfirmationsController` which override and/or inherits from their original devise controllers.
+
+## Omniauth
+
+Google OmniAuth is used to authenticate users via Google's OAuth 2.0 API. This means users can sign in to your application using their Google account credentials. This simplifies the sign-in process for users, as they don't need to remember another username/password.
+
+### Configuration
+
+In the provided codebase, Google OmniAuth is configured in the same file as devise `config/initializers/devise.rb`.
+
+```
+config.omniauth :google_oauth2,
+  Rails.application.credentials.dig(Rails.env.to_sym, :google, :client_id),
+  Rails.application.credentials.dig(Rails.env.to_sym, :google, :client_secret),
+  scope: "email, profile"
+```
+
+1. The `client_id` and `client_secret` should be setup via the [Google API console](https://console.developers.google.com/) and then stored in the credentials file. You can follow [this tutorial](https://fwuensche.medium.com/how-to-use-google-oauth-on-rails-c6e07047e4fb) to set up your google client keys on the **Google API console** or get them from your admin if the keys are already setup.
+2. The google oauth callback creates a **User** record using the `User.from_omniauth(data)` method that can be found in the `User::Omniauthable` concern. You may customize this method according to the requirements of your app.