improve README.md: detailed installation + result screenshot
Revert "fix README: jirac can be executed from any directory"

This reverts commit bcbb7f5cd00aee9f7efa680ed79901a9b985ff61.
Merge branch 'work_on_documentation' of github.com:lesaint/jirac into lesaint-work_on_documentation
Merge branch 'lesaint-work_on_documentation'