Changes to Annals/Letter to Henning 1220

• 2021-06-06 14:34:54 UTC (Hans Georg Schaathun): Philippus Niger
• 2021-05-30 10:45:43 UTC (Hans Georg Schaathun): fixed signature
• 2021-05-30 10:44:36 UTC (Hans Georg Schaathun): Added letter
• 2021-05-30 07:38:32 UTC (Hans Georg Schaathun): Renamed file. Added link