Added a draft of the networkcache.

src/zc/buildout/networkcache.txt

+Using buildout with network-cache
+=================================
+
+NOTE: You need to install the networkcached into your system to run this
+      test, otherwise it will not be possible to get proper results.
+
+The buildout itself must be able to cache everything which is downloadble
+into an external and universal cache server.
+
+Such feature is not enabled by default, but you can enable defining the
+option called ```network-cache```.
+
+   >>> write(sample_buildout, 'buildout.cfg',
+   ... '''
+   ... [buildout]
+   ... network-cache = http://127.0.0.1:5001/
+   ... find-links = %(link_server)s
+   ... parts = eggs
+   ...
+   ... [eggs]
+   ... recipe = zc.recipe.egg:eggs
+   ... eggs = demo ==0.2
+   ... ''' % globals())
+
+   >>> print system(buildout)
+   Installing eggs.
+   Getting distribution for 'demo==0.2'.
+   The url is not cached yet: http://localhost/demo-0.2-py2.6.egg
+   Got demo 0.2.
+   Getting distribution for 'demoneeded'.
+   The url is not cached yet: http://localhost/demoneeded-1.2c1.zip
+   Got demoneeded 1.2c1.
+   ...
+
+   >>> from glob import glob
+   >>> from os.path import join
+   >>> for egg in glob(join(sample_buildout, 'eggs', 'demoneeded*.egg')):
+   ...     remove(sample_buildout, 'eggs', egg)
+
+   >>> print system(buildout)
+   Updating eggs.
+   Getting distribution for 'demoneeded'.
+   Downloading from network cache http://127.0.0.1:5001/105e4a9ba3f9c7f45b46cc8c47ac2d2ca6440ae564538090c6d390a8959dd03e
+   Got demoneeded 1.2c1.
+   ...