add upgrade notes

docs/release-notes/release-notes-future.md

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+Joinmarket-clientserver future:
+===============================
+
+
+Upgrading 
+=========
+
+To upgrade: run the `install.sh` script as mentioned in the README. When prompted to overwrite the directory `jmvenv`, accept.
+
+A new wallet format has been introduced. Old wallets require conversion. In order to convert your existing wallet to the new format you can use the included conversion tool at `scripts/convert_old_wallet.py`.
+
+usage:
+
+    python convert_old_wallet.py full/path/to/wallets/wallet.json
+
+This will place the newly converted `wallet.jmdat` file in the existing joinmarket `wallets/` directory. The wallet name will be adopted accordingly if it differs from `wallet`.
+
+
+Notable changes
+===============
+
+
+Credits
+=======
+
+Thanks to everyone who directly contributed to this release -
+
+
+And thanks also to those who submitted bug reports, tested and otherwise helped out.