README.md: add usage for locally installed koy

User might not want to pollute their global environment with npm install koy -g and choose to install locally via npm install koy. That way, users need to invoke koy this way:

node_modules/.bin/koy FILENAME.md

or alternatively,

npx koy FILENAME.md

npx is a utility of NPM. It will check the node_modules in the current directory for the executable. If not found, it will automatically download the package.

By the way, Carlo author's team members send their regards.