Update repo documentation

[isaisabel]

Updates repository documentation:

• Add sinformation about ATT&CK in STIX 2.1 and links to that repository
• Replaces old ATT&CK blurb with newer and more accurate ATT&CK blurb
• Moves STIX section to after the CAPEC section, since CAPEC is content on the repo and STIX is context for how it's represented
• Improvements to introductory paragraph for consistency with attack-stix-data

[rpiazza]

@isaisabel - looks fine to me