Private state update metadata and migration (#404)

(backport from release-1.4)
Private state update metadata and migration

Signed-off-by: Lucas Saldanha <lucas.saldanha@consensys.net>

docs/Private-Txns-Migration.md

@@ -0,0 +1,36 @@
+# Private Transactions Migration 
+
+Hyperledger Besu v1.4 implements a new data structure for private state storage that is not backwards compatible. 
+A migration will be performed when starting v1.4 for the first time to reprocess existing private transactions 
+and re-create the private state data in the v1.4 format. 
+
+**Important**  
+
+All nodes with existing private transactions will be migrated to the new private state storage 
+when upgrading to v1.4. It is not possible to upgrade to v1.4 without migrating. 
+
+## How to migrate 
+
+**Important** 
+
+As a precaution (that is, resyncing should not be required), ensure your Hyperledger Besu database is backed-up 
+or other Besu nodes in your network are available to resync from if the migration does not complete as expected.  
+
+We recommend that all nodes in a network do not upgrade and migrate at once. 
+
+To migrate, add the `--privacy-enable-database-migration` flag to your Besu command line options. The migration starts 
+automatically when this flag is specified. If you have existing private transactions and do not specify this flag, 
+v1.4 will not start.  
+
+After the migration starts, logs display the progress.  When the migration is complete, Besu continues 
+starting up as usual. 
+
+## During migration  
+
+Do not stop the migration running once it has started. If the migration is stopped, you will need to restore
+your Besu database from backup and restart the migration process. 
+
+## Migration support 
+
+If you have a long running network with a large volume of private transactions and/or would like to discuss
+the migration process with us before upgrading, contact us at support@pegasys.tech