add reference to KAMG

The documentation of KAMG should include a reference explaining what it is detail.

Is this an implementation following the paper [Notay and Vassilevski, 2007]?

That's the one. Must have clear enough. Feel free to add it to the documentation.

mentioned in merge request !24 (merged)

Status changed to closed by commit 4477d811

mentioned in commit 9bf96e07

mentioned in commit 4477d811