decrease the heading level of usage sections in the readme
it makes more sense to link to the jar since that was renamed and the shell wrapper added in a previous release.
add some additional description to what's going on in the build steps for mdm itself.
add note about portability to README.

it seems like a reasonable point to emphasize given that A) it's a core design goal that has informed many structural choices and B) it's embarassing how many other package managers out there more or less require a package manager for a mortal to install them.
additional comments on portability.