Added reference to confluence documentation from the developers guide
This closes
@@ -2569,6 +2569,7 @@ Contributions can be provided either by creating a patch:
and attaching that patch to a ticket, or by generating a Pull Request.
+For further details regarding contributions, refer to the relevant sections of the link:https://cwiki.apache.org/confluence/display/NIFI/Contributor+Guide#ContributorGuide-providingCodeOrDocumentationContributionProvidingcodeordocumentationcontributions[contributor guide].
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