Add master branch build status to `README.md`.
Add master branch build status to `README.md`.

POV-Ray/povray,wfpokorny/povray,wfpokorny/povray,POV-Ray/povray,wfpokorny/povray,POV-Ray/povray,eblanca/povray,LeForgeron/povray,LeForgeron/povray,wfpokorny/povray,wfpokorny/povray,LeForgeron/povray,BentSm/povray,LeForgeron/povray,c-lipka/povray,LeForgeron/povray,BentSm/povray,POV-Ray/povray,LeForgeron/povray,eblanca/povray,eblanca/povray,wfpokorny/povray,wfpokorny/povray,eblanca/povray,LeForgeron/povray,c-lipka/povray,c-lipka/povray,POV-Ray/povray,POV-Ray/povray,POV-Ray/povray,BentSm/povray,c-lipka/povray,BentSm/povray,LeForgeron/povray