Improve main README file

Right now, our main readme file is very poor:
https://github.com/apache/tomee/blob/master/README.md

I think we should improve it a bit by adding the following information:

• Project description
• Downloads
• Contribution guidelines
• Links to docs with examples or other relevant information
• Links to tools used by the project
• Any other relevant stuff

This may look like a duplication from the website. We can and should link to pieces of information there. On the other hand, people may find the project directly on Github and should have a minimal piece of information to get them started.