Update CONTRIBUTING.md
Update CONTRIBUTING.md

removed mention of Trello for bug tracking
annapowellsmith/openpresc,ebmdatalab/openprescribing,annapowellsmith/openpresc,ebmdatalab/openprescribing,annapowellsmith/openpresc,ebmdatalab/openprescribing,ebmdatalab/openprescribing,annapowellsmith/openpresc