Document and explain DB migration issue (#1079)

* add explanation

Signed-off-by: Stefan Klotz <stefan.klotz@bosch.io>

* add hyperlinks

Signed-off-by: Stefan Klotz <stefan.klotz@bosch.io>

* rephrase

Signed-off-by: Stefan Klotz <stefan.klotz@bosch.io>

* rephrase

Signed-off-by: Stefan Klotz <stefan.klotz@bosch.io>

MIGRATION.md

@@ -1,3 +1,4 @@
+
 # hawkBit Migration Guides
 ## Release 0.2
 ### Configuration Property changes
@@ -22,3 +23,19 @@
 ## Milestone 0.3.0M6
 ### Configuration Property changes
 - hawkbit.server.security.dos.maxTargetsPerManualAssignment has changed to hawkbit.server.security.dos.maxTargetDistributionSetAssignmentsPerManualAssignment
+
+## Upgrade from Master Branch (after 0.3.0M6) to 0.3.0M7
+Due to changes in the DB migration scripts within PR [#1017](https://github.com/eclipse/hawkbit/pull/1017) the Hawkbit will not start up if one of the following cases is true:
+- DB2 database is used
+- MSSQL database is used and the sp_action table is not empty
+- PostgreSql database is used and the sp_action table is not empty
+
+The script was fixed with PR [#1061](https://github.com/eclipse/hawkbit/pull/1061).
+
+In case you upgrade from 0.3.0M6 to 0.3.0M7 there is no issue. But if you have built the Hawkbit from the master branch between PR [#1017](https://github.com/eclipse/hawkbit/pull/1017) and PR [#1061](https://github.com/eclipse/hawkbit/pull/1061), use PostgreSQL or MSSQL and upgrade to 0.3.0M7, it will fail at startup with the message: `Validate failed: Migration checksum mismatch for migration version 1.12.16`
+
+This can be fixed by adapting the schema_version table of the database. The checksum field of the entry with the version 1.12.16 has to be changed (mind the minus):
+- -1684307461 for MSSQL
+- -596342656 for PostgreSql
+
+Example for MSSQL: `UPDATE schema_version SET checksum=-1684307461 WHERE version='1.12.16'`