Add comment on fixing bugs to CONTRIBUTING.md

Summary:
Based on Amaury's comments in Dev Slack channel
Explain that development process prioritizes fixing bugs when found, above other tasks

Test Plan: Amaury should check that the wording reflects how he wants the process to work

Reviewers: deadalnix, schancel, jasonbcox, #bitcoin_abc

Reviewed By: jasonbcox, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1294