build + doc: update module install path, update README.md

We were installing modules into `$libdir/flux/modules/cmb/` but as we didn't end up prefixing module names with cmb, we can drop that directory from the path.  Also when that was in place it became evident after a `make install` that the wreck subdir was not using the configure.ac defined values for install directiores, so fix that.

An overview section is added to the README.md.