buildbot.yml: Change bot user to 'scons' and explain how to override
buildbot.yml: Change bot user to 'scons' and explain how to override

timj/scons,timj/scons,andrewyoung1991/scons,andrewyoung1991/scons,timj/scons,andrewyoung1991/scons,andrewyoung1991/scons,andrewyoung1991/scons,timj/scons,timj/scons,andrewyoung1991/scons,timj/scons,andrewyoung1991/scons,andrewyoung1991/scons,andrewyoung1991/scons,timj/scons,timj/scons,timj/scons