Add link to Overwinter info page

src/consensus/upgrades.cpp

@@ -22,7 +22,7 @@ const struct NUInfo NetworkUpgradeInfo[Consensus::MAX_NETWORK_UPGRADES] = {
     {
         /*.nBranchId =*/ 0x5ba81b19,
         /*.strName =*/ "Overwinter",
-        /*.strInfo =*/ "TBD",
+        /*.strInfo =*/ "See https://z.cash/upgrade/overwinter.html for details.",
     }
 };