docs/papers: Import protect-community-repositories-nsdi2016.pdf

This change is part is of tuf's commit:

Add Diplomat paper

Author: vladdd <[email protected]>
Origin: theupdateframework/python-tuf@e943048
Relate-to: theupdateframework/python-tuf#1380
URL: #159
Forwarded: #160
Signed-off-by: Philippe Coval <[email protected]>

Author: vladimir-v-diaz