Add licenses to Markdown

[Fokko]

I noticed that we ignore markdown files:

iceberg-rust/.licenserc.yaml

Line 26 in 106be6f

- '**/*.md'

Let's also add the markdown, similar to PyIceberg: https://github.com/apache/iceberg/blob/master/python/mkdocs/docs/api.md

[Xuanwo]

Wow, I didn't know we could add a license header in Markdown!