Add install section to README

[deeplook]

Because it's a standard good practice to tell people how to install something. ;)

[MacroGrove]

Yeah, I'm trying to install it now, but I can't find information instructions anywhere online.

[gaby]

Install via pip

pip3 install backoff

Library is already published to pypi.org

[bgreen-litl]

What needs to happen here is we should move the full documentation out of the README (to something like readthedocs) and the README should get streamlined and include information like this. The docs used to be appropriately small, but as the API has somewhat expanded, we should optimized the README for Getting Started, and development etc.

[ranggakd]

What needs to happen here is we should move the full documentation out of the README (to something like readthedocs) and the README should get streamlined and include information like this. The docs used to be appropriately small, but as the API has somewhat expanded, we should optimized the README for Getting Started, and development etc.

Can we get the update on this? Documentation is really important for this.