Update link to "additional configuration" documentation

[AObuchow]

Description

When the 'additional configuration' documentation was changed from Markdown to AsciiDoc, the link on the README wasn't updated, and thus leads to a 404-page on GitHub.

How To Reproduce

Go to DWO's README's additional configuration section and click on the link to see additional documentation.

Expected behavior

The user is linked to https://github.com/devfile/devworkspace-operator/blob/main/docs/additional-configuration.adoc

Additional context