Add a Contributor Guide

CONTRIBUTING.md

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+# Contributor Guide
+
+## Environment setup and workflow
+- Clone the repo
+- Create a fresh python environemt with `.binder/requirements.txt` file
+- Open `jupyter lab` or `jupyter notebook` (so that jupytext extension can take effect)
+- Then right-click on a MyST markdown notebook, choose `Open With` -> `Notebook`
+or `Jupytext Notebook` (more info [here](https://nasa-navo.github.io/navo-workshop/00_SETUP.html#handling-notebooks-in-myst-markdown-format)).
+
+> Note: If you'd like to work with this repository in an editor/IDE of your choice,
+make sure that you have relevant extensions to parse MyST markdown notebooks to
+interactive python notebook (ipynb) and to reflect the changes made there to markdown
+format:
+>- VSCode: doesn't seem to have a reliable extension to open md files as notebook, so you may have to manually pair them using jupytext in terminal.
+>- Other editors: ...
+
+## Updating an existing notebook (.md format)
+- Open it in jupyter notebook or lab, jupytext extension should be installed (it's already listed in `requirements.txt` file mentioned above)
+- Right click on a md notebook file and "open with" notebook format. 
+- Edit it as you would a regular notebook and once saved the changes should 
+reflect in md
+- Make sure only content changes appear in md diff, not the frontmatter, before 
+comitting
+- Make a PR once ready (TODO: explain how like forking etc.)
+
+## Adding a new notebook (from .ipynb format)
+To add an ipynb notebook you were working on to this repository, convert that to
+MyST markdown using:
+
+```python
+jupytext --to md:myst notebook.ipynb 
+```
+
+Then add it to `tutorials/` and also add its path in a relevant TOC in `index.md`.