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A link to the forum thread to the tracking milestone RFC - even if outdated, it provides a starting point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!
Adding requested changes

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).

Period after "User Forum", and 'you can..' should be capitalized ("Be sure to check the User Forum. You can...").
Update README.md

Updated logo
Merge pull request #580 from claudiordgz/readme-mod

Add basic new user guidance to README.md #475