The only document currently released that people are sometimes referring to as "The SDK" is the manual for the OMSI Editor..... You can do a lot with the Editor, but it's not an SDK as such.
Marcel & Rudiger (the developers) are currently producing an (full) SDK as I understand it.
The OMSI Editor Manual (English & German) now comes with Version 1.01 of the game, or I think you can also download a standalone version from the Aerosoft Manual.
D.