Improve readme introduction

This improved introduction was started by @trotzig[1] who wanted to
introduce the app a little better with a couple of use-cases.

[1]: https://github.com/trotzig/diffux/commit/138daccb2fe2b0747ea6911f4507ae39d92fbdec

Change-Id: Ib09bf6e328e1b51f32f1d2af747f0f9f120ef8ea