Add basic new user guidance to README.md #475

Replace the welcome document GitHub file blob link with the web hosted version
A link to the forum for discussion
A link to the forum thread to the tracking milestone RFC - even if outdated, it provides a starting point
Add the Cinder logo!
I think a better link to use would be `http://libcinder.org/images/logo.png` - it is a static url, rather than one generated by doxygen (for the dead `dev` branch at that).