Document clock convenience field.

service/README.md

@@ -173,6 +173,7 @@ To create a new network with a random last six digits safely and atomically, you
 <tr><td>creationTime</td><td>integer</td><td>Time network was created in ms since epoch</td><td>no</td></tr>
 <tr><td>revision</td><td>integer</td><td>Network config revision number</td><td>no</td></tr>
 <tr><td>memberRevisionCounter</td><td>integer</td><td>Current value of network revision counter (incremented after every member add or revision)</td><td>no</td></tr>
+<tr><td>clock</td><td>integer</td><td>Current clock in ms since epoch (for convenience)</td><td>no</td></tr>
 <tr><td>authorizedMemberCount</td><td>integer</td><td>Number of authorized members</td><td>no</td></tr>
 <tr><td>relays</td><td>[object]</td><td>Array of network-specific relay nodes (see below)</td><td>yes</td></tr>
 <tr><td>ipLocalRoutes</td><td>[string]</td><td>Array of IP network/netmask entries corresponding to networks routed directly via this interface (e.g. 10.0.0.0/8 to route 10.0.0.0 via this interface)</td></tr>
@@ -243,6 +244,7 @@ IP related fields apply only to Ethernet frames of type IPv4 or IPV6. Otherwise
 <table>
 <tr><td><b>Field</b></td><td><b>Type</b></td><td><b>Description</b></td><td><b>Writable</b></td></tr>
 <tr><td>nwid</td><td>string</td><td>16-digit hex network ID</td><td>no</td></tr>
+<tr><td>clock</td><td>integer</td><td>Current clock in ms since epoch (for convenience)</td><td>no</td></tr>
 <tr><td>address</td><td>string</td><td>10-digit hex ZeroTier address</td><td>no</td></tr>
 <tr><td>authorized</td><td>boolean</td><td>Is member authorized?</td><td>yes</td></tr>
 <tr><td>activeBridge</td><td>boolean</td><td>This member is an active network bridge</td><td>yes</td></tr>