[FAB-3058] CONTRIBUTING improvements
[FAB-3058] CONTRIBUTING improvements

Make inclusion of JIRA link mandatory, add improved guidance for
change requests.

Change-Id: I5cd2e39e4fb4b3df486fdfa3a349dd328eb6972d
Signed-off-by: Christopher Ferris <b3041279349bcb20974a27fd07c65415cf2b925e@us.ibm.com>

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