Update Getting Started Doc to include composer commands for altering existing composer.json projects

For the .gitignore section, I think we can add something like this:

cd /path/to/repo
find . -name "/recipes*" >> .gitignore

------------

To add a repository, it's

composer config repositories.<name> <type> <url>

Example:

composer config repositories.drupal composer https://packages.drupal.org/8

This will automatically inject into composer.json under repositories like this:

"repositories": {
    "drupal": {
        "type": "composer",
        "url": "https://packages.drupal.org/8"
    }
}

------------

To update the allow-plugins section for the unpack command:

The general syntax is:

composer config allow-plugins.<plugin-name> true

For our specific case:

composer config allow-plugins.ewcomposer/unpack true

Merged, thanks!