Consider expectation-setting in README.md

[alexfdrover]

Consider adding a section to README.md that outlines the expected level of background knowledge on various topics for best results. E.g:

"This book assumes you have an intermediate understanding of Rust, and are at least passingly familiar with computer networking. The code samples assume you are running Linux, and will not work on other operating systems without modification. It is assumed you are using Rust (XX) or newer - backwards compatibility is not guaranteed."