XD-596 Add CONTRIBUTING.md file

Jira: https://jira.springsource.org/browse/XD-596
XD-596 Code review fix
Quick commit for feedback
Working code for local named channels, lacking parametrization & clean() method
Used a boolean instead of the Creator Strategy object, as this only applies to Local; Introduced an aliasHint though (named TBC)
Fixing compile error in test
Fix javadoc to reflect actual behavior of new LocalChannelRegistry
XD-594 Add list, delete shell commands for metrics
  - Added commands for Field value counter, aggregate counter, gauge and rich gauge metrics
    (Excluding the list command for aggregate counter)
  - Added metrics controller integration tests for delete
Merge pull request #182 from ghillert/XD-596

XD-596 Add CONTRIBUTING.md file