Add Legistar client links to the README

[anaerobeth]

I am working on replicating this project for the OpenImpact replication marathon and thought it would be useful to add a Legistar client list to the README.

Covers:

• Add a list of Legistar client links to the README

Notes:

• Links were confirmed active as of July 30, 2015.
• The google doc is public and editable by anyone who has the link. If preferred, I can link to a gist or a view-only document.

Data sources:

• Legistar client list from Legistar-people
• Google Maps