Added documentation for executor authorization.
Added documentation for executor authorization.

This patch adds documentation regarding the implicit authorization
of HTTP executors.

Review: https://reviews.apache.org/r/58625/

reneploetz/mesos,dforsyth/mesos,kaysoky/mesos,neilconway/mesos,gsantovena/mesos,jpeach/mesos,reneploetz/mesos,jpeach/mesos,gsantovena/mesos,neilconway/mesos,gsantovena/mesos,andschwa/mesos,asamerh4/mesos,neilconway/mesos,jpeach/mesos,zmalik/mesos,chhsia0/mesos,reneploetz/mesos,abudnik/mesos,asamerh4/mesos,verizonlabs/mesos,Gilbert88/mesos,dforsyth/mesos,asamerh4/mesos,chhsia0/mesos,reneploetz/mesos,dforsyth/mesos,jpeach/mesos,neilconway/mesos,kaysoky/mesos,kaysoky/mesos,abudnik/mesos,zmalik/mesos,reneploetz/mesos,abudnik/mesos,gsantovena/mesos,neilconway/mesos,kaysoky/mesos,kaysoky/mesos,asamerh4/mesos,Gilbert88/mesos,andschwa/mesos,zmalik/mesos,andschwa/mesos,dforsyth/mesos,verizonlabs/mesos,Gilbert88/mesos,verizonlabs/mesos,verizonlabs/mesos,gsantovena/mesos,chhsia0/mesos,asamerh4/mesos,abudnik/mesos,verizonlabs/mesos,neilconway/mesos,dforsyth/mesos,asamerh4/mesos,abudnik/mesos,gsantovena/mesos,chhsia0/mesos,abudnik/mesos,dforsyth/mesos,kaysoky/mesos,jpeach/mesos,chhsia0/mesos,Gilbert88/mesos,jpeach/mesos,neilconway/mesos,verizonlabs/mesos,andschwa/mesos,kaysoky/mesos,asamerh4/mesos,andschwa/mesos,verizonlabs/mesos,chhsia0/mesos,andschwa/mesos,andschwa/mesos,abudnik/mesos,zmalik/mesos,zmalik/mesos,reneploetz/mesos,zmalik/mesos,jpeach/mesos,zmalik/mesos,Gilbert88/mesos,dforsyth/mesos,reneploetz/mesos,chhsia0/mesos,Gilbert88/mesos,gsantovena/mesos,Gilbert88/mesos