Update README.rst to include upstream planning etherpad
Update README.rst to include upstream planning etherpad

Small updates to the README, including adding instructions on how to
generate a `config.json` using stackviz-export and local subunit+dstat
files. Also includes a link to the upstream planning etherpad, which
will be used for ideas about getting StackViz working with data from
devstack-gate runs.

Change-Id: I206cb5ea6331eaf005b8c9c43a1e7b93e2d77a92

openstack/stackviz,openstack/stackviz,openstack/stackviz