Hm, i know in the Horus Heresy books as well as the art book (with summary explanation of events) about it it is mentioned. Dark Mechanicus opens with a tale of how one of the Martian Knights witnesses the landing of the Emperor's Space Ship on Mars, and his machine is "healed" by the Emperor's touch (a flaw which was beyond the adepts of Mars to repair). In the book there are several references in passing to the alliance. If you follow the link in my post though at the bottom you can see where the lexicanum got their info from.
As for how it developped: good question. I've no idea. They got a seat on the high lords of terra post heresy, but other then that i doubt much has changed. They did loose a ton of knowledge during the heresy so they are in a worse condition technologically speaking then they were before the heresy.