There was a great deal of speculation about what other potential crimes he may have committed, and what Mueller had up his sleeve, but that is just speculation.
Here are all three documents, along with Flynn's post guilty plea statement: Flynn Plea Agreeement
I suspect I conflated what I was reading on Conservative Treehouse with the actual documents. CT has done a retrospective on their coverage that includes links to many of the original articles today, here's the link: Flynn’s Mueller Cooperation – Two Turks Charged With Conspiracy as Agents of Foreign Government…
We do know from Mueller's sentencing recommendation that Flynn has been providing substantial help on an unrelated matter, the details of which were sealed and redacted.
In addition, the Addendum to this memorandum describes the defendant’s assistance to the government. Because the Addendum includes sensitive information about ongoing investigations, the government is seeking to partially seal the Addendum, and has filed publicly a redacted version of the document that excludes the sensitive information.
Given today's indictments it's easy to guess that he was ratting out his Turkish contacts.