add pre-commit to CONTRIBUTING.txt
add pre-commit to CONTRIBUTING.txt

markshao/pytest,hackebrot/pytest,nicoddemus/pytest,The-Compiler/pytest,nicoddemus/pytest,pfctdayelise/pytest,davidszotten/pytest,tomviner/pytest,alfredodeza/pytest,RonnyPfannschmidt/pytest,pytest-dev/pytest,txomon/pytest,Akasurde/pytest,tomviner/pytest,The-Compiler/pytest,ddboline/pytest