Fix various links in README.md (#14630)
Fix various links in README.md (#14630)

- Committer's guide was broken
- Some links pointed to old url
- Added reference to scheduler docs
bolkedebruin/airflow,apache/incubator-airflow,lyft/incubator-airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,lyft/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,dhuang/incubator-airflow,apache/airflow,mistercrunch/airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,sekikn/incubator-airflow,danielvdende/incubator-airflow,apache/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,apache/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,bolkedebruin/airflow,apache/airflow,dhuang/incubator-airflow,nathanielvarona/airflow,sekikn/incubator-airflow,danielvdende/incubator-airflow,sekikn/incubator-airflow,nathanielvarona/airflow,mistercrunch/airflow,sekikn/incubator-airflow,apache/airflow,apache/airflow,lyft/incubator-airflow,cfei18/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,dhuang/incubator-airflow,apache/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow