Add note about supported series/models.

documentation-overview.html

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         <h1>Documentation Overview</h1>
         This documentation describes how to patch Asterisk to support Cisco SIP IP phones as well as how to configure the various features available on those devices. These IP phones require additional proprietary extensions to the SIP protocol to operate correctly and are only designed to operate with Cisco Unified Call Manager.<br>
         <br>
+        Supported devices are the <b>6900</b> series, <b>7800</b> series, <b>7900</b> series, <b>8800</b> series, <b>894X</b> models and <b>9900</b> series.<br>
+        <br>
         Questions may be sent to <a href="mailto:[email protected]"><span class="icon">mail</span> [email protected]</a>. Source code for the project (patch, utilities and documentation) is also available on <a href="https://github.com/usecallmanagernz"><span class="icon">open_in_browser</span> GitHub</a>.<br>
         <br>
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