Add composer.json

[strarsis]

It would be nice to have a composer.json in this repository for installing the plugin via composer.

[noxify]

Workaround:

"repositories": [
    {
      "type": "composer",
      "url": "https://wpackagist.org"
    },
    {
      "type": "package",
      "package": {
        "name": "marcusig/gutenberg-section-block",
        "version": "1.0.0",
        "type": "wordpress-plugin",
        "source": {
          "url": "https://github.com/marcusig/gutenberg-section-block",
          "type": "git",
          "reference": "master"
        }
      }
    }
],
"require": {
    "marcusig/gutenberg-section-block": "*",
},