Fixed copy and paste for link reference
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)
* Removed references to "checkout and building" until thats defined
* Add links to GitHub issues
Merge remote-tracking branch 'upstream/master'
BATCH-2267: added ItemReader and ItemWriter implementations for elasticsearch
BATCH-2267: fixed formatting of README.md
Merge remote-tracking branch 'upstream/master'
Updated javadoc.
Fixed potential NullPointer.
Updated to Spring Batch 3.0.1
Merge remote-tracking branch 'origin/master'