Write documentation or include it from CPP Driver

[wprzytula]

Once #134 enabled Scylla docs in the repo, we can introduce documentation to the CPP Rust Driver.
It should consist of the following parts / cover the following topics:

Overview

Building, Installation

Testing

Basic Usage Examples

API

This part can likely be copied from the CPP Driver, as the API is nearly the same. From a quick look at the CPP Driver, the API docs are generated from cassandra.h using Doxygen, so we should be able to mimic it.

Driver Features

Those should be taken from the CPP Driver and adjusted for differences originating from Rust Driver underneath.