README Update/Overhaul

[bquintopng]

Hi! My name is Ben and I'm doing a pull request on a repo for a university assignment.

I made a few improvements to the original README such as:

• Adjusted formatting
• Added headings to organize the page
• Reorganized features moving "community" and "documentation" to the bottom of the list. (I believed these were less important than other "features" which are the main selling points of ember)
• Added an "installation" section to allow users to quickly earn how to install
• Added a "getting started" section on how to create a new ember app

I know a lot of this information is already in the guides, but I thought adding them to the readme would allow for users to quickly find the necessary information to begin with ember without needing to navigate the docs.

Thanks!