readme: Convert to Markdown

* Add .md extension so GitHub recognises it as Markdown.
* Add <pre> wrappers for the code snippets.
* Backtick endpoints.
* Turn mentioned file names into links to their contents.
* Fix typo in file name ("server_translations.js" -> "server_translation.js").
Merge pull request #7 from Krinkle/readme

readme: Convert to markdown