Update README.md

Dead link.  Revised instructions for downloading the 2 Cisco SDK DLLs:
    The Cisco SDK redistributables are available at
        http://solutionpartner.cisco.com/web/physical-security/documentation
    1. Download: "VSM 7.0.1 SDK with Examples"
    2. Unzip: VSM 7.0.1 SDK with Examples.zip
    3. Find in: VSM7.0.1SDK\WebServicesDLLs
        VSOMWebService.dll
        VSOMWebService.XmlSerializers.dll