Add requires.io info to README; add alt tags to other badges
Add requires.io info to README; add alt tags to other badges

Bachmann1234/diff-cover,Bachmann1234/diff-cover,benpatterson/diff-cover,edx/diff-cover,mblayman/diff-cover,mblayman/diff-cover,ezequielo/diff-cover,ezequielo/diff-cover,benpatterson/diff-cover