Mention cryptg as an optional dependency

optional-requirements.txt

@@ -0,0 +1 @@
+cryptg

readthedocs/extra/basic/installation.rst

@@ -33,6 +33,13 @@ If you don't have root access, simply pass the ``--user`` flag to the pip
 command. If you want to install a specific branch, append ``@branch`` to
 the end of the first install command.
 
+By default the library will use a pure Python implementation for encryption,
+which can be really slow when uploading or downloading files. If you don't
+mind using a C extension, install `cryptg <https://github.com/Lonami/cryptg>`__
+via ``pip`` or as an extra:
+
+    ``pip3 install telethon[cryptg]``
+
 
 Manual Installation
 *******************

requirements.txt

@@ -0,0 +1,2 @@
+pyaes
+rsa

setup.py

@@ -149,7 +149,10 @@ def main():
                 'telethon_generator', 'telethon_tests', 'run_tests.py',
                 'try_telethon.py'
             ]),
-            install_requires=['pyaes', 'rsa']
+            install_requires=['pyaes', 'rsa'],
+            extras_require={
+                'cryptg': ['cryptg']
+            }
         )