chore: add release notes for composite command for whitelist

doc/release-notes-6100.md

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+## Remote Procedure Call (RPC) Changes
+
+### Improved support of composite commands
+
+Dash Core's composite commands such as `quorum list` or `bls generate` now are compatible with a whitelist feature.
+
+For example, whitelist `getblockcount,quorumlist` will let to call commands `getblockcount`, `quorum list`, but not `quorum sign`
+
+Note, that adding simple `quorum` in whitelist will allow to use all kind of `quorum...` commands, such as `quorum`, `quorum list`, `quorum sign`, etc
+
+
+### The new RPCs are:
+
+- `quorum signplatform` This RPC is added for Platform needs. This composite command let to limit quorum type for signing by platform. It is equivalent of `quorum sign <platform type>`.
+