ddev v1.18 now supports mutagen explcitly

[rfay]

If you're interested in this project, we'd love to have you try out ddev's new explicit mutagen integration in v1.18+. See https://ddev.com/ddev-local/supercharge-your-ddev-performance-with-mutagen/ and https://ddev.readthedocs.io/en/latest/users/performance/#using-mutagen

[cweagans]

Added a link to this issue from the readme for visibility!