README update for sequelize v6

[lukashroch]

Update readme

• how to upgrade to sequelize v6 & sequelize-typescript v2
• outline breaking changes
• sequelize-typescript v1 with v5

[DaPfeifa]

I hope this can be merged soon.
Otherwise you always run into problems when migrating to Sequelize v6.

[lukashroch]

I've squashed the changes and added a note about current state of implementation, so it's more clear what currently works.

• less strict sequelize V6 works quite fine - this is equivalent to what we had so far
• more strict sequelize introduced in v6.2+ is not yet implemented, will need more work

@RobinBuschmann if this could be merged, it would be much appreciated. I should have done the README update along with the code change, so users knew current state and what needs to be done. Thanks!