[docs] Add telegram supergroup to CONTRIBUTING.md
[docs] Add telegram supergroup to CONTRIBUTING.md

Summary: n/t

Test Plan: n/a

Reviewers: deadalnix, Mengerian, jasonbcox, matiu, #bitcoin_abc

Reviewed By: deadalnix, jasonbcox, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1027

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