Container: upgrading notices and steps for the next ocis release

[mmattel]

This container lists upgrading steps/notes for the upcoming ocis release.

• Service Accounts: [full-ci] Service Accounts ocis#6427 (auth-service),
  also see New service: auth-service #592
  For existing installations you need to set the OCIS_SERVICE_ACCOUNT_ID and OCIS_SERVICE_ACCOUNT_SECRET envvars. For new installations ocis init sets the values and no envvars are needed.

• HC: pods priority

• HC: Master compatibility

• HC: add hostaliases

• HC: add webfinger

• HC: pods priority

• HC: Remove unused STORAGE_USERS_PURGE_TRASH_BIN_USER_ID

@kobergj @dragonchaser, @wkloucek fyi, pls add more content as dev moves on