Revise Introduction in Quick Start Guide

Thanks to @mstanichenko, Libtrading has a great Quick Start Guide that will help anyone interested in using the library or contributing to the project.

Few days ago I started revising the Quick Start Guide lightly, tightening the prose and cleaning up the markup. Soon I noticed, though, that this work is actually quite time-consuming, not to mention subjective. To gauge interest and incite discussion, I opened this pull request. It contains revisions to the Introduction section of the document.