make *.md linguist detectable.

[kehiy]

No description provided.

[alexgleason]

What does that do?

[kehiy]

@alexgleason currently all of this repo is *.md files. github can't determine and show it by default; there is no top-used language, and it looks like an empty repository from outside. adding this file will show md as the language used here.

for example bolt repo.

in their pr to add this file they have some more reasons for that as well which we can consider(?).