Add release notes

doc/release-notes/release-notes-5.0.0-rc1.md

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+zend v5.0.0-rc1
+=========
+
+This version is expected to deprecate on Mainnet at block **#XXXXXXX**, which will occur on **Month Xth XXXX at approximately XX:XX PM UTC**. Please, update to a newer version before Month XXth.
+
+Zend `4.1.1` is going to deprecate on Mainnet at block **#1502084**, which will occur on **February 6th 2024 at approximately 12:00 PM UTC**. Please, update to a `v5.0.0` version before February 6th.
+
+Zend `5.0.0` is going to deprecate on Mainnet at block **#xxxxxxx**, which will occur on **xxx xxth, 2024 at approximately xx:xx PM UTC**.
+
+A **hard fork** is going to activate on Mainnet at block **#xxxxxxx**, which will occur on **xxx xxth, 2024 at approximately xx:xx PM UTC**
+
+The same **hard fork** will activate on testnet at block **#1404200**, which will occur on **January 16th at approximately 12:00:00 UTC**.
+
+## Important Notes
+
+### Shielded pool removal
+- PR [#628](https://github.com/HorizenOfficial/zen/pull/628) implements [ZenIP-42207](https://github.com/HorizenOfficial/ZenIPs/blob/master/zenip-42207.md), which introduces a hard fork fully disabling the shielded pool from `zend`. After the hard fork, it will no longer be possible to submit and process transactions involving shielded (z-) addresses, thus transparent-to-shielded, shielded-to-shielded and shielded-to-transparent transactions will be completely disallowed.
+- PR [#628](https://github.com/HorizenOfficial/zen/pull/628) as a consequence of [ZenIP-42207](https://github.com/HorizenOfficial/ZenIPs/blob/master/zenip-42207.md), the following RPC commands
+    - `z_sendmany`
+    - `z_mergetoaddress`
+
+    have been partially disabled, as after the hard fork they are only allowed to submit transparent-to-transparent transactions (check inline documentation for additional details).
+
+## New Features and Improvements
+
+## Bugfixes
+
+## Minor Changes
+- PR [#628](https://github.com/HorizenOfficial/zen/pull/628) all the RPC commands interacting with the shielded pool - hence, all the RPC commands starting with `z_` or `zc_` (for instance `z_getbalanace` or `zc_raw_joinsplit`) - are now *deprecated*. A warning message has been included in the help body of these commands.
+
+## Contributors
+
+* [@JackPiri](https://github.com/JackPiri)
+* [@a-petrini](https://github.com/a-petrini)
+* [@drgora](https://github.com/drgora)
+* [@ptagl](https://github.com/ptagl)
+* [@mantle0x](https://github.com/mantle0x)
+
+Special thanks to [@mantle0x](https://github.com/mantle0x) for the first contribution to Zen!