Add release notes for version 4.2.0.0

doc/release-notes/release-notes-4.2.0.0.md

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+# Metrix Core v4.2.0.0
+
+## Major Update
+
+This is a major protocol upgrade in response to an issue discovered within the (v2) DGP contracts.
+This upgrade will enable usage of newly created DGP contracts after consensus is reached.
+
+Failure to upgrade after activation of MIP6 will result in disconnection from the greater Metrix network.
+
+## Bug Reporting
+
+Please report bugs using the issue tracker at github: https://github.com/thelindaproject/metrix/issues
+
+## How to Upgrade
+Shut down Metrix, wait until it has completely shut down (which might take a few minutes
+for older versions), then just copy over the appropriate metrixd file.
+
+## What's Changed
+- Changed target timespan for BIP9 signaling from 2016 to 6720 (MIP4)
+- Added start block height for BIP9 signaling
+- Softfork using BIP9 consensus vote
+- Bugfix to ensure governance winner validation occurs in contract after the DPGv3 migration.
+- Disable legacy validation method once MIP6 activates, this will reduce CPU used by the daemon after the activation block.
+- Increment version 4.2.0.0
+- New protocol version 70026
+- Implementation of [MIP4](https://github.com/TheLindaProjectInc/MIPs/blob/main/mip-4.md)
+- Implementation of [MIP5](https://github.com/TheLindaProjectInc/MIPs/blob/main/mip-5.md)
+- Implementation of [MIP6](https://github.com/TheLindaProjectInc/MIPs/blob/main/mip-6.md)