Updated 2 links from mitre.org to their github urls
Updated 2 links from mitre.org to their github urls

Updated 2 links from mitre.org to their github urls + some minor copy edits +  deleted a sentence referencing new releases being posted on the STIX Discussion List + updated STIX Discussion List mailto to a stix@mitre.org mailto in the feedback requested sentence
STIXProject/java-stix-validator,STIXProject/java-stix-validator,STIXProject/java-stix-validator