Created Installing Telethon (markdown)

Installing-Telethon.md

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+```bash
+pip install telethon
+```
+
+If you get something like `"SyntaxError: invalid syntax"` or any other error while installing, it's probably because `pip` defaults to Python 2, which is **not supported**. Use `pip3` instead.
+
+If you already have the library installed, upgrade with `pip install --upgrade telethon`.
+
+## Manual installation
+
+1. Install the required `pyaes` ([GitHub](https://github.com/ricmoo/pyaes)|[PyPi](https://pypi.python.org/pypi/pyaes)) and `rsa` ([GitHub](https://github.com/sybrenstuvel/python-rsa/)|[PyPi](https://pypi.python.org/pypi/rsa/3.4.2)) modules: `sudo -H pip install pyaes rsa`
+2. Clone Telethon's GitHub repository: ``git clone https://github.com/LonamiWebs/Telethon.git``
+3. Enter the cloned repository: ``cd Telethon``
+4. Run the code generator: ``python3 setup.py gen_tl``
+5. Done!
+
+To generate the documentation, `cd docs` and then `python generate.py`.
+
+## Optional dependencies
+If you're using the library under ARM (or even if you aren't), you may want to install `sympy` through `pip` for a substantial speed-up when generating the keys required to connect to Telegram (you can of course do this on desktop too). See [issue #199](https://github.com/LonamiWebs/Telethon/issues/199) for more.
+
+If `libssl` is available on your system, it will also be used wherever encryption is needed.
+
+If neither of these are available, a pure Python callback will be used instead, so you can still run the library wherever Python is available!