Hi Cris,
I'm happy to hear you are interested. MM0 is currently under active development so lots of things are in flux, but the best resources for MM0 are currently the specification
https://github.com/digama0/mm0/blob/master/mm0.md
and other files in the
https://github.com/digama0/mm0/ repo, and the MM0 thread on this list (
https://groups.google.com/forum/#!topic/metamath/raGj9fO6U2I). The mailing list thread has some high level description of the progress. The mm0.md spec is a specification of the .mm0 file format, which is sufficient for being able to read the examples in the examples/ directory. The proofs are not contained in the mm0 files, but all the information needed to understand the statement of the target theorem(s) are.
The main verifier if you want to play with this yourself is the mm0-hs verifier in Haskell. There is another spec in
https://github.com/digama0/mm0/blob/master/mm0-hs/README.md that gives the implementation-defined aspects of MM0 specific to the haskell verifier. In particular, the .mmu proof format is described there, so you can also use that to read .mmu files in the examples/ directory.
I'm documenting on the fly, so if you have any suggestions of areas I should focus on to make anything clearer please let me know.
Mario Carneiro