Include class relationship diagrams in network stack documentation.

net/docs/README.txt

+To generate the output HTML from an input markdown file, execute the
+command:
+
+        PYTHONPATH=../../third_party python -m markdown <input>.md > output.html

[mmenke, 2016/04/04 21:19:26]

This is not platform independent.

[Randy Smith (Not in Mondays), 2016/04/04 21:26:50]

I was going to ask you about that in the detailed review :-}.  I presume it's ok
to just put a Windows version in here too?  I think it would be something like:

set PYTHONPATH=..\..\third_party
python -m markdown -f <output>.html <input>.md

That an ok plan?  (I haven't tested the above, but I expect it would work and
I'll make sure it's tested before landing.)

[mmenke, 2016/04/04 21:31:12]

SGTM, and that does indeed work on Windows.

[Randy Smith (Not in Mondays), 2016/04/04 21:44:50]

Excellent.  In it goes.

+
+(This command line assumes that the net/docs directory is the current
+directory; if that's not the case, adjust the path to src/third_party
+to be accurate from whatever directory the command is executed from.)
+
+The diagrams included in the network stack documentation were
+generated with Graphviz, and both source (.dot) and output (.svg) are
+included in the repository.  If graphviz is installed, the output may
+be regenerated from the source via:
+
+        dot dot -Tsvg <name>.dot > <name>.svg