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

Looks good. Thanks @thejimbirch

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

Updated the docs. I don't think we should call the recipe unpack plugin optional, just give people simple and consistent commands they can use to get going. The less cognitive load, the better.

We desperately need this to be up to date, so I'm hoping we can merge this sooner rather than later.

Merged, thanks!