Document built-in variables in deployment template configuration

Both `MODULES.foo` and `${MODULEDIR<../relative-path>}` appear to be supported and replaced by `iotedgedev genconfig` but neither pattern is documented.

It is unclear:

1. What the value of `$MODULEDIR`?
2. PR #391 also appear to show it's customizable with MODULES_PATH which is undocumented
3. What is required to have `foo` recognized in `MODULES.foo`?
4. Which mechanism should be used / is preferred?

More broadly - why are there two ways to access modules, and why is the $MODULEDIR treated as a pseudo-variable with different syntax from all the other environment variables? The non-standard syntax for path delcarations is confusing at best.

If it's attempting to construct a path to a module, why not just let the user construct it themselves? `${MODULEDIR}/../relative-path` would maintain consistency with all other variables used in the templates and follow standard convention, instead of requiring something specific to IoT Edge.