README needs updating

[rlratzel]

The README is in desperate need of updating in order for this repo to be adopted by more projects/users.

It needs the following examples:

• Basic use by a "project" script that overrides the default config. This was the initial use case, where a project like cuGraph or cuML would, for example, have a MNMG test running script that used the mg-tools scripts to setup the MNMG environment, but had directories, conda env names, etc. specific to their project that overrode the default config before calling the mg-tools scripts.
• Using the scripts interactively from the CLI

Since the scripts are intended to be a "toolbox", they are meant to be called individually as part of a larger user workflow (these scripts could eventually be packaged as a conda install). Because of that, the README also needs the following documentation:

• Description of the script hierarchy (in an ASCII tree-like format) which includes every script.
• Section containing a brief one-line description of what each script does.