add a hint that the spec was machine generated

[mimoo]

Github can detect machine generated file, and display them differently.

For HTML it's here: https://github.com/github/linguist/blob/master/lib/linguist/generated.rb#L708

no implementation exists for markdown though

in both cases it sounds like adding something like that at the top of the file would be enough:

<!-- This is an automatically generated file, do not edit manually. -->