Sometimes picojson's .mkdn isn't easy to see.

[YuseiIto]

Visual Studio Code(by default) can't recognition .mkdn file as markdown.

Please change the README's filename extention from .mkdn to .md .
You know, It's a VSCode's ploblem. But filename extention "md" is more popular than "mkdn", I think changeing filename extention is not bad idea.
Please fix the small ploblem to be better.