Include class relationship diagrams in network stack documentation.

net/docs/README.txt

Index: net/docs/README.txt
diff --git a/net/docs/README.txt b/net/docs/README.txt
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+To generate the output HTML from an input markdown file on Unix
+machines, execute the command:
+
+  PYTHONPATH=../../third_party python -m markdown -f <output>.html  <input>.md 
+
+On Windows machines, execute:
+
+  set PYTHONPATH=..\..\third_party
+  python -m markdown -f <output>.html <input>.md
+
+(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