Update README.md and add CONTRIBUTING.md

README.md:
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* 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
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* CLA project was left as is (SIE doesn't have a specific entry either)
* BATCHEXT is assumed the topic branch prefix (may need updating when actually created)
* Removed references to "checkout and building" until thats defined
* Add links to GitHub issues