Config XML

Magento2 SysConfig XML Example

Required Files:
app/code/Vendor/Modulename/etc/adminhtml/system.xml
app/code/Vendor/Modulename/etc/acl.xml

system.xml:
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../Config/etc/system_file.xsd">
    <system>
        <!-- Add new Tab -->
        <tab id="vendor" translate="label" sortOrder="300">
            <label>Vendor Extension</label>
        </tab>
        <section id="helloworld" translate="label" type="text" sortOrder="140" showInDefault="1" showInWebsite="1" showInStore="1">
            <label>Helloworld</label>
            <tab>vendor</tab>
            <!-- resource tag name which we have to defined in the acl.xml -->
            <resource>Vendor_Helloworld::config_helloworld</resource>
        </section>
        <group id="general" translate="label" type="text" sortOrder="1" showInDefault="1" showInWebsite="1" showInStore="1">
            <label>General Options</label>
            <field id="active" translate="label" type="select" sortOrder="1" showInDefault="1" showInWebsite="1" showInStore="1">
                <label>Enabled</label>
                <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
            </field>
        </group>
    </system>
</config>

acl.xml
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/Acl/etc/acl.xsd">
    <acl>
        <resources>
            <resource id="Magento_Backend::admin">
                <resource id="Magento_Backend::stores">
                    <resource id="Magento_Backend::stores_settings">
                        <resource id="Magento_Config::config">
                            <!-- this resource id we can use in system.xml for section -->
                            <resource id="Vendor_Helloworld::config_helloworld" title="Helloworld Section" sortOrder="80" />
                        </resource>
                    </resource>
                </resource>
            </resource>
        </resources>
    </acl>
</config>

Source: http://stackoverflow.com/questions/32614392/magento-2-system-xml-in-custom-module