Create rollback.md (#1133)

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migrate/v0_16/rollback.md

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+# Kava-9 Rollback Instructions
+
+In the event that the Kava-9 relaunch is unsuccessful, we will restart the Kava-8 chain using the last known state.
+
+In order to restore the previous chain, the following data must be recovered by validators:
+
+- The database that contains the state of the previous chain (in ~/.kvd/data by default)
+- The priv_validator_state.json file of the validator (in ~/.kvd/data by default)
+
+If you don't have the database data, the Kava developer team or another validator will share a copy of the database via Amazon s3 or a similar service. You will be able to download a copy of the data and verify it before starting your node.
+If you don't have the backup priv_validator_state.json file, you will not have double sign protection on the first block. If this is the case, it's best to consult in the validator discord before starting your node.
+
+## Restoring state procedure
+
+1. Stop your node
+
+```sh
+kava stop
+```
+
+2. Copy the contents of your backup data directory back to the $KVD_HOME/data directory. By default this is ~/.kvd/data.
+
+```sh
+# Assumes backup is stored in "backup" directory
+rm -rf ~/.kvd/data
+mv backup/.kvd/data ~/.kvd/data
+```
+
+3. Install the previous version of kava
+
+```sh
+# from kava directory
+git checkout v0.15.2
+make install
+## verify version
+kvd version --long
+```
+
+4. Start kvd process
+
+```sh
+### be sure to remove --halt-time flag if it is set
+sudo systemctl daemon-reload
+sudo systemctl start kvd
+```