Add remaining project files (exclude ignored folders)

payment/midtrans/maintaining.md

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+> Warning: This note is for developer/maintainer of this package only
+
+## Updating Package
+
+- Make your changes
+- Update `version` value on `composer.json`
+- Update library version header on `ApiRequestor.php`
+- Commit and push changes to Github master branch
+- Create a [Github Release](https://github.com/Midtrans/midtrans-php/releases) with the target version
+- Github Release and Master Branch is automatically synced to [the Packagist version](https://packagist.org/packages/midtrans/midtrans-php)
+  - Because of configured integration on Github & Packagist
+  - To edit integration config sign in with the Midtrans Packagist Account