Update README.md and add CONTRIBUTING.md

README.md:
* Make link to Spring Batch project on spring.io for more context as we already have the git repo
* General Markdown cleanup
* Maven command cleanup
* Fix commercial support link
* Add links to GitHub issues
    
Added CONTRIBUTING.md from Spring Batch project as a template
* CLA project was left as is (SIE doesn't have a specific entry either)
* Removed references to "checkout and building" until thats defined
* Add links to GitHub issues