[AIRFLOW-XXX] Update Contributing Guide - Git Hooks (#4120)
[AIRFLOW-XXX] Update Contributing Guide - Git Hooks (#4120)

- changes pre-commit example to use methods
- adds activating virtual env for python to run things like flake8 locally
- changes pre-commit file to use set -e command to instantly exit if any non-zero error occurs
- changes flake8 call to lint the repo instead of not only the changes files


lyft/incubator-airflow,mistercrunch/airflow,spektom/incubator-airflow,malmiron/incubator-airflow,jgao54/airflow,dhuang/incubator-airflow,jgao54/airflow,Fokko/incubator-airflow,sekikn/incubator-airflow,apache/airflow,wileeam/airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,artwr/airflow,dhuang/incubator-airflow,mrkm4ntr/incubator-airflow,spektom/incubator-airflow,nathanielvarona/airflow,adamhaney/airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,fenglu-g/incubator-airflow,r39132/airflow,apache/incubator-airflow,danielvdende/incubator-airflow,airbnb/airflow,bolkedebruin/airflow,fenglu-g/incubator-airflow,mistercrunch/airflow,airbnb/airflow,DinoCow/airflow,mrkm4ntr/incubator-airflow,mrkm4ntr/incubator-airflow,cfei18/incubator-airflow,mtagle/airflow,cfei18/incubator-airflow,adamhaney/airflow,nathanielvarona/airflow,apache/incubator-airflow,apache/incubator-airflow,mtagle/airflow,wileeam/airflow,lyft/incubator-airflow,Fokko/incubator-airflow,r39132/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,airbnb/airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,wileeam/airflow,adamhaney/airflow,wooga/airflow,wileeam/airflow,mistercrunch/airflow,Fokko/incubator-airflow,apache/airflow,spektom/incubator-airflow,danielvdende/incubator-airflow,apache/airflow,Fokko/incubator-airflow,airbnb/airflow,DinoCow/airflow,artwr/airflow,adamhaney/airflow,r39132/airflow,cfei18/incubator-airflow,mtagle/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,sekikn/incubator-airflow,apache/airflow,jgao54/airflow,artwr/airflow,cfei18/incubator-airflow,bolkedebruin/airflow,malmiron/incubator-airflow,nathanielvarona/airflow,spektom/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,sekikn/incubator-airflow,DinoCow/airflow,wooga/airflow,wooga/airflow,mistercrunch/airflow,mrkm4ntr/incubator-airflow,fenglu-g/incubator-airflow,bolkedebruin/airflow,malmiron/incubator-airflow,artwr/airflow,wooga/airflow,malmiron/incubator-airflow,fenglu-g/incubator-airflow,Acehaidrey/incubator-airflow,mtagle/airflow,apache/airflow,DinoCow/airflow,apache/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,r39132/airflow,Acehaidrey/incubator-airflow,jgao54/airflow,lyft/incubator-airflow,dhuang/incubator-airflow,sekikn/incubator-airflow,bolkedebruin/airflow