Add breadcrumb README files in various directories

[drewoldag]

We currently have a breadcrumb README in the .../docs/notebooks directory when a new project is hydrated. I'm suggesting that we add more and perhaps use those READMEs as a place to add a little more documentation.

i.e. for the one in the .../docs/notebooks directory, something like:

This directory contains all the notebooks that will be rendered in ReadTheDocs. 
You'll have to be sure to add the file name to the ../notebooks.rst file.
You may also need to update the requirements.txt file if your notebooks have requirements not specified in pyproject.toml.

For more information look here: https://lincc-ppt.readthedocs.io/en/latest/practices/sphinx.html#python-notebooks
Or if you still have questions contact us: https://lincc-ppt.readthedocs.io/en/latest/source/contact.html

We could do something like this for:

• .github
• benchmarks
• src
• tests