Add backporting suggestion to CONTRIBUTING.md
Summary:
Point to the backporting.md document if people are looking
for something to work on.
Test Plan: Read it
Reviewers: deadalnix, jasonbcox, #bitcoin_abc
Reviewed By: deadalnix, jasonbcox, #bitcoin_abc
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2183