doc: Add release notes

doc/release-notes-18077.md

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+P2P and network changes
+-----------------------
+
+- Added NAT-PMP port mapping support via [`libnatpmp`](https://miniupnp.tuxfamily.org/libnatpmp.html)
+
+Command-line options
+--------------------
+
+- The `-natpmp` option has been added to use NAT-PMP to map the listening port. If both UPnP
+and NAT-PMP are enabled, a successful allocation from UPnP prevails over one from NAT-PMP.