Updated advanced contributing guide.
Updated advanced contributing guide.

Removed the instruction to create a reviewboard account
from the advanced contributing guide, since this instruction
became impossible to follow for contributors.

Review: https://reviews.apache.org/r/70129/

gsantovena/mesos,chhsia0/mesos,gsantovena/mesos,jpeach/mesos,dforsyth/mesos,kaysoky/mesos,chhsia0/mesos,Gilbert88/mesos,abudnik/mesos,jpeach/mesos,reneploetz/mesos,reneploetz/mesos,kaysoky/mesos,gsantovena/mesos,dforsyth/mesos,jpeach/mesos,Gilbert88/mesos,abudnik/mesos,reneploetz/mesos,abudnik/mesos,dforsyth/mesos,chhsia0/mesos,Gilbert88/mesos,jpeach/mesos,reneploetz/mesos,dforsyth/mesos,kaysoky/mesos,chhsia0/mesos,gsantovena/mesos,chhsia0/mesos,jpeach/mesos,chhsia0/mesos,dforsyth/mesos,gsantovena/mesos,kaysoky/mesos,reneploetz/mesos,abudnik/mesos,dforsyth/mesos,jpeach/mesos,chhsia0/mesos,Gilbert88/mesos,reneploetz/mesos,abudnik/mesos,kaysoky/mesos,jpeach/mesos,dforsyth/mesos,kaysoky/mesos,abudnik/mesos,reneploetz/mesos,kaysoky/mesos,gsantovena/mesos,abudnik/mesos,Gilbert88/mesos,Gilbert88/mesos,gsantovena/mesos,Gilbert88/mesos