I do not know what is SDK, but it is clear for me that it would help to modders to make mods. I heard that people hoped to have SDK after CS is released. So I asked about release of modding instruments on the forum of GSC in thread "Developers answering questions". I asked when SDK or something like modding instruments will be issues if ever.
I received answer from Drakon v palto that the problem is as follows: to make such an instrument with references and instructions and user's manual there should be a department or at least team for that purpose only. Unfortunately in Russia developing teams of different games are very small and they have not enough people for development of the game even, not mentioning mod-instruments.
So I guess there will be no sdk, whatever it is.