doc: Add release notes for 29775

Author: fjahr

doc/release-notes-29775.md

@@ -0,0 +1,10 @@
+Testnet4/BIP94 support
+-----
+
+Support for Testnet4 as specified in [BIP94](https://github.com/bitcoin/bips/blob/master/bip-0094.mediawiki)
+has been added. The network can be selected with the `-testnet4` option and
+the section header is also named `[testnet4]`.
+
+While the intention is to phase out support for Testnet3 in an upcoming
+version, support for it is still available via the known options in this
+release.