Polishing README.md

- Corrected English phrasing throughout
- Display screenshot on the page instead of a link
- Clarification as to what 'libtoxav' is
- Numeration for compilation steps instead of an unsorted list
- Separated compilation instructions into steps and notes
- Added links for dependencies
Merge pull request #155 from theGeekPirate/patch-1

Polishing README.md