Add preliminary sphinx documentation

docs/source/installation.md

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+# Installation
+
+🐸TTS supports python >=3.6 <=3.9 and tested on Ubuntu 18.10, 19.10, 20.10.
+
+## Using `pip`
+
+`pip` is recommended if you want to use 🐸TTS only for inference.
+
+You can install from PyPI as follows:
+
+```bash
+pip install TTS  # from PyPI
+```
+
+By default, this only installs the requirements for PyTorch. To install the tensorflow dependencies as well, use the `tf` extra.
+
+```bash
+pip install TTS[tf]
+```
+
+Or install from Github:
+
+```bash
+pip install git+https://github.com/coqui-ai/TTS  # from Github
+```
+
+## Installing From Source
+
+This is recommended for development and more control over 🐸TTS.
+
+```bash
+git clone https://github.com/coqui-ai/TTS/
+cd TTS
+make system-deps  # only on Linux systems.
+make install
+```
+
+## On Windows
+If you are on Windows, 👑@GuyPaddock wrote installation instructions [here](https://stackoverflow.com/questions/66726331/