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