README: add section about reporting security vulnerabilities
README: add section about reporting security vulnerabilities

It was pointed out by @davidillsley that we don't point this out
anywhere in our docs.

circleci/dex,Calpicow/dex,wolfeidau/dex,chub/coreos-dex,coreos/dex,chub/coreos-dex,rithujohn191/dex,chub/coreos-dex,Calpicow/dex,Calpicow/dex,wolfeidau/dex,coreos/dex,ericchiang/dex,ericchiang/dex,coreos/dex,wolfeidau/dex,rithujohn191/dex,circleci/dex,circleci/dex,ericchiang/dex,rithujohn191/dex