CONTRIBUTING.md : recommend ccache

Summary:
Both florian and I were late to the ccache thing and when we finally
started using it, we were very pleased with the results. For new contibutors
this is a helpful tip.

Test Plan: none

Reviewers: #bitcoin_abc, deadalnix, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2554