Make the README file better and distribute it along the binary.

[carstene1ns]

Currently, the README file is mainly targeted at building the Player executable.

It should:

• be polished and also contain some information only available in the wiki currently
• use the Markdown format to allow advanced rendering
• be distributed along the binary in the downloads on https://easyrpg.org (for some platforms it could be renamed to .txt to avoid confusion Windows)