Added networking and http-api slack channels to work group docs.
Added networking and http-api slack channels to work group docs.

Review: https://reviews.apache.org/r/49457/

dforsyth/mesos,verizonlabs/mesos,andschwa/mesos,jpeach/mesos,zmalik/mesos,kaysoky/mesos,neilconway/mesos,abudnik/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,dforsyth/mesos,kaysoky/mesos,yuquanshan/customizedMesos,craimbert/mesos,chhsia0/mesos,asamerh4/mesos,craimbert/mesos,neilconway/mesos,gsantovena/mesos,jpeach/mesos,kaysoky/mesos,kaysoky/mesos,craimbert/mesos,andschwa/mesos,craimbert/mesos,dforsyth/mesos,chhsia0/mesos,chhsia0/mesos,reneploetz/mesos,chhsia0/mesos,yuquanshan/customizedMesos,gsantovena/mesos,Gilbert88/mesos,yuquanshan/customizedMesos,abudnik/mesos,kaysoky/mesos,neilconway/mesos,Aman-Jain-14/customizedMesos,jpeach/mesos,asamerh4/mesos,verizonlabs/mesos,dforsyth/mesos,andschwa/mesos,reneploetz/mesos,neilconway/mesos,Aman-Jain-14/customizedMesos,dforsyth/mesos,verizonlabs/mesos,verizonlabs/mesos,jpeach/mesos,asamerh4/mesos,andschwa/mesos,verizonlabs/mesos,gsantovena/mesos,abudnik/mesos,abudnik/mesos,neilconway/mesos,kaysoky/mesos,reneploetz/mesos,Gilbert88/mesos,craimbert/mesos,jpeach/mesos,zmalik/mesos,chhsia0/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,dforsyth/mesos,reneploetz/mesos,Aman-Jain-14/customizedMesos,Aman-Jain-14/customizedMesos,dforsyth/mesos,reneploetz/mesos,Gilbert88/mesos,neilconway/mesos,verizonlabs/mesos,asamerh4/mesos,jpeach/mesos,chhsia0/mesos,Gilbert88/mesos,Gilbert88/mesos,zmalik/mesos,chhsia0/mesos,asamerh4/mesos,abudnik/mesos,Gilbert88/mesos,andschwa/mesos,yuquanshan/customizedMesos,zmalik/mesos,gsantovena/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,reneploetz/mesos,gsantovena/mesos,Gilbert88/mesos,zmalik/mesos,andschwa/mesos,yuquanshan/customizedMesos,neilconway/mesos,zmalik/mesos,jpeach/mesos,craimbert/mesos,abudnik/mesos,asamerh4/mesos,asamerh4/mesos,craimbert/mesos,andschwa/mesos,kaysoky/mesos,zmalik/mesos,gsantovena/mesos,reneploetz/mesos,abudnik/mesos,gsantovena/mesos