External file for rewrite rules

myRewrites.config

<rules>
    <rule name="some rule">
        <!-- rule details here --->
    </rule>
</rules>

web.config

<configuration>
    <system.webServer>
        <rewrite>
            <rules configSource="config\myRewrites.config">
        </rewrite>
    </system.webServer>
</configuration>