Make the module compatible with composer

[capynet]

• The module needs to be able to be added as dependency on a project via composer require drupal/OURMODULE
• The module itself needs a composer.json with "paypal/rest-api-sdk-php" as dependency.

[fran]

I think the problem is fixed since Drupal use wikimedia/composer-merge-plugin. Adding this code to composer.json in the project the library will be added automatically.

"merge-plugin": {
            "include": [
                "core/composer.json",
                **"modules/contrib/DrupalPayPalRecurrringPayment/paypal_sdk/composer.json"**
            ],
            "recurse": true,
            "replace": false,
            "merge-extra": false
        },

We should only update the README.md file with a note to add the line.