Feature: Add 'readme' command.

[doosuu]

The CLI is missing a readme command which iterates over all packages and merges their README.md files into one big readme which summarizes the capabilities of the template repository.

The idea is to output a docs folder which contains all documentation from all used packages with one entry point markdown file which links all of these READMEs together:

Folder structure might look like this:

<root>/
├── doc/
│   └── index.md
│   └── devenv-runtimes/
│       └── index.md
│   └── devenv-github-integration/
│       └── index.md

Main index.md may look like this:

# SampleApp

## The project consists of the following packages

* devenv-runtimes (link-to-respective-index-md)
  * <list-of-exposed-programs>
  * <list-of-exposed-variables>
* devenv-github-integration (link-to-respective-index-md)
  * <list-of-exposed-programs>
  * <list-of-exposed-variables>