Field In A Theme

Carbon Fields allows you to define custom fields inside your theme if you have theme-specific fields that you do not intend to distribute separately:
Execute 
git clone https://github.com/htmlburger/carbon-field-template.git --single-branch --depth 1 includes/my-field in your theme dir
Delete the 
includes/my-field/.git directory
Add the following to your theme's composer.json:
"autoload": {
    "files": [
        "./includes/my-field/field.php"
    ],
    "psr-4": {
        "Carbon_Field_YOURFIELDNAME\\": "./includes/my-field/core/"
    }
}
Call 
composer dumpautoload
Browse to the 
includes/my-field directory
Call 
npm install
Edit 
includes/my-field/webpack.config.js and adjust the 
const root variable to point to the directory of Carbon Fields (e.g. 
const root = path.resolve(__dirname, '../../vendor/htmlburger/carbon-fields');)
Call 
npm run build
You now have a local custom field running from your theme
If the field is theme-specific and will not be distributed separately feel free to delete the 
composer.json and 
composer.lock files.