Document coding style with EditorConfig

Summary:
Style follows Nasqueron one:

• 4 spaces for indentation
• UNIX-style EOL (\n)
• Encoding in UTF-8
• Trailing whitespaces should be removed
• Files should end with a new line (EOL at EOF)

Test Plan: Tested in Atom and PhpStorm

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2811