Mention the existence of the devfile-converter library in migrating-to-devfile-v2

[themr0c]

The existence of the devfile-converter library [1] is probably worth mentioning in the migration page [2].
[1] npmjs.com/package/@eclipse-che/devfile-converter
[2] devfile.io/docs/devfile/2.1.0/user-guide/migrating-to-devfile-v2.html