Add 2-FA instructions for non-technical users (#580)

docs/Using-Mastodon/2FA.md

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+# 2-Factor Authentication
+
+2-Factor Authentication is a security mechanism that requires you to enter a computer generated code from your phone every time you log into Mastodon.
+
+We highly recommend that you set up 2-factor authentication as it prevents malicious users from logging into your account if they obtain your password.
+
+## Warning
+
+If you lose access to your 2-factor authentication (such as by losing your phone or performing a factory reset) and you do cannot log in, you will not be able to access your account and will need to contact an instance admin to remove 2-factor authentication from your account.
+
+## Setup
+
+1. Open your [settings page](https://mastodon.social/settings/two_factor_auth) and navigate to the Two-factor Authentication page
+2. Press the big blue "Enable" button that appears on the right ![screenshot](screenshots/2fa/enable.png)
+3. Follow instructions below to install an authenticator for your smartphone
+
+## Android
+
+__Recommended Application:__ [Google
+Authenticator](https://play.google.com/store/apps/details?id=com.google.android.apps.authenticator2)
+
+4. Download the above application on your phone
+5. Open the "Authenticator" app
+6. Press the + button in the bottom right-hand corner
+7. Press "Scan a barcode"
+8. Line up the black and white QR code with the target box that appears on your camera
+9. Now, whenever you log in to Mastodon, open the Authenticator app and enter the 6 digit code that appears above the "mastodon.social (email address)" text
+
+## iPhone
+
+__Recommended Application:__ iPhone: [Authenticator by Matt
+Ruben](https://itunes.apple.com/us/app/authenticator/id766157276?mt=8)
+Authenticator](https://play.google.com/store/apps/details?id=com.google.android.apps.authenticator2)
+
+4. Download the above application on your phone
+5. Open the "Authenticator" app
+6. Press the + button in the bottom right-hand corner
+7. Authenticator should prompt you for access to your camera- hit "OK"
+8. Line up the black and white QR code with the target box that appears on your camera
+9. Now, whenever you log in to Mastodon, open the Authenticator app and enter the 6 digit code that appears above the "mastodon.social (email address)" text
+
+# Disabling 2-factor Authentication
+
+1. Go to [the 2-factor authentication settings page](https://mastodon.social/settings/two_factor_auth)
+2. Press the big blue "Disable" button underneath your QR code ![disable button screenshot](screenshots/2fa/disable.png)