Doc: Add Multi-factor authentication guide

readme/apps/mfa.md

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+# Multi-factor authentication
+
+Multi-Factor Authentication (MFA), also known as Two Factor Authentication (2FA), enhances your account security by requiring more than just a password to log in. 
+
+
+## How to enable MFA
+
+- [Access your profile](https://joplincloud.com/users/me)
+- Go to the **Multi-factor authentication** section
+- Click the link to **Enable Multi-Factor Authentication**
+- Follow the instructions shown in the page
+
+## How to disable MFA
+
+- [Access your profile](https://joplincloud.com/users/me)
+- Go to the **Multi-factor authentication** section
+- Click the link **I want to disable it** and follow the instructions
+
+## Recovery codes
+
+When you enable MFA, Joplin Cloud will show you a list of codes that you should save somewhere safe. Each code can only be used once but you can generate new codes from your profile page.
+
+If you cannot generate your authentication codes because you have lost access to your smartphone, for example, you will need to use one of the recovery codes to login.
+
+## I forgot to copy my recovery codes, how do I find them?
+
+- [Access your profile](https://joplincloud.com/users/me)
+- Go to the **Multi-factor authentication** section
+- Click the **Recovery codes** link
+- Follow the instructions shown in the page
+
+## How do I generate new recovery codes?
+
+- [Access your profile](https://joplincloud.com/users/me)
+- Go to the **Multi-factor authentication** section
+- Click the **Recovery codes** link
+- After finishing the process you should see your list of recovery codes and a button at the bottom to generate new codes
+
+> The new codes will invalidate the old ones, so remember to save them.
+
+## How to login with my recovery code
+
+- Proceed to Joplin Cloud login page and follow the usual login process
+- When prompted for an authentication code click in the link **Use a recovery code** bellow the input field
+- Add your code to login