README.md: replace CircleCI badge with TC
README.md: replace CircleCI badge with TC

Closes #10513.

paperstreet/cockroach,petermattis/cockroach,maxlang/cockroach,maxlang/cockroach,veteranlu/cockroach,veteranlu/cockroach,maxlang/cockroach,vaibhavarora/cockroach,glycerine/cockroach,maxlang/cockroach,paperstreet/cockroach,veteranlu/cockroach,paperstreet/cockroach,veteranlu/cockroach,paperstreet/cockroach,paperstreet/cockroach,maxlang/cockroach,petermattis/cockroach,vaibhavarora/cockroach,petermattis/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,glycerine/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,paperstreet/cockroach,maxlang/cockroach,petermattis/cockroach,vaibhavarora/cockroach,petermattis/cockroach,glycerine/cockroach,veteranlu/cockroach,glycerine/cockroach,glycerine/cockroach,glycerine/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,maxlang/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,glycerine/cockroach,petermattis/cockroach,petermattis/cockroach,vaibhavarora/cockroach,maxlang/cockroach,veteranlu/cockroach,petermattis/cockroach,glycerine/cockroach