Created Two Factor Authorization (markdown)

Two-Factor-Authorization.md

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+If you have Two Factor Authorization (from now on, 2FA) enabled on your account, calling `.sign_in` will raise a `SessionPasswordNeededError`. When this happens, just `.sign_in()` again with a `password=`:
+
+```python
+from telethon.errors import SessionPasswordNeededError
+
+client.sign_in(phone)
+try:
+    client.sign_in(code=input('Enter code: '))
+except SessionPasswordNeededError:
+    client.sign_in(password=input('Enter password: '))
+```
+
+## Enabling 2FA
+
+If you don't have 2FA enabled, but you would like to do so through Telethon, take as example the following code snippet:
+
+```python
+import os
+from hashlib import sha256
+from telethon.tl.functions import account
+from telethon.tl.types.account import PasswordInputSettings
+
+new_salt = client(account.GetPasswordRequest()).new_salt
+salt = new_salt + os.urandom(8)  # new random salt
+
+pw = 'secret'.encode('utf-8')  # type your new password here
+hint = 'hint'
+
+pw_salted = salt + pw + salt
+pw_hash = sha256(pw_salted).digest()
+
+password_new = PasswordInputSettings(new_salt=salt, new_password_hash=pw_hash, hint='hint')
+result = client(account.UpdatePasswordSettingsRequest(
+    current_password_hash=salt,
+    new_settings=PasswordInputSettings(
+        new_salt=salt,
+        new_password_hash=pw_hash,
+        hint=hint
+    )
+)
+```
+
+Thanks to [issue 259](https://github.com/LonamiWebs/Telethon/issues/259) for the tip!