Update Contributing.md file with new DAG standards

[gabrielwol]

I think it might be beneficial to start packing this block of code (updating Python path, setting DAG's default args, getting the doc_md, etc.) which is shared in almost all DAGs in one initialization function for DAGs. and maybe also add it to the Contributing.md file
What do you think? Maybe a new issue and PR for this

Originally posted by @tahaislam in #804 (comment)

[gabrielwol]

Include info on html comment tags to include readme as per:
#804 (comment)

[tahaislam]

As part of updating the documentation of adding new DAGs, we should also include what's needed to be modified in test scripts
#797 (comment)