Why .md files does not have Copyright Licenses?

[pritidesai]

@jerop brought up that we do not have copyright licenses in any of the files under docs folder:

https://github.com/tektoncd/pipeline/pull/4600/files#r815156707

I found this weird and wondering why the licenses are missing 🤔

We have added such licenses to the Apache project I have contributed before and they were added since the foundation mandated them. I am not sure what are the guidelines here.

[vdemeester]

Note: I think that by default the main [LICENSE][./LICENSE] applies when there is no license header in files.