Update Contribution guide to point to RFC process
Update Contribution guide to point to RFC process

The doc still directs people to open an epic or story on JIRA to
propose new features. This change directs people to use the RFC
process instead.

Signed-off-by: Arnaud J Le Hors <336b2f4fb14e0a6aba0291ba81894e387297584e@us.ibm.com>

stemlending/fabric,stemlending/fabric,manish-sethi/fabric-sidedb,stemlending/fabric,hyperledger/fabric,manish-sethi/fabric,manish-sethi/fabric-sidedb,stemlending/fabric,manish-sethi/fabric-sidedb,manish-sethi/fabric-sidedb,jimthematrix/fabric,manish-sethi/fabric,manish-sethi/fabric-sidedb,manish-sethi/fabric-sidedb,jimthematrix/fabric,manish-sethi/fabric-sidedb,hyperledger/fabric