Add description of .mllib ordering requirements

[jfehrle]

It appears that if module B depends on module A, then "B" should appear in .mllib after "A". However this essential fact isn't mentioned in the documentation. This suggests that circular dependencies are not supported: if B depends on A then A cannot depend on B.

If there are other rules on the content of .mllib files, it would be helpful to document.

If someone helps me get the right info I will prepare a change request.