Add Accelerate book to Bitcoin ABC reading list
Add Accelerate book to Bitcoin ABC reading list

Summary: Adds another relevant link to CONTRIBUTING.md

Test Plan: Check the link, make sure it works

Reviewers: deadalnix, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4847

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