start a developer notes

Author: Jonathanlyj

DEVELOPER_NOTES.md

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+## Notes for PnetCDF-python developers
+---
+
+### Note on configuring Read the Docs tool for user guide generation
+ * Doc files
+   * `.readthedocs.yaml` -- The script that controls commands that run before installation of pnetcdf-python (under `pre-install:`) and installation (under `python:`)
+   * `/docs/requirements.txt` -- Python dependencies required for doc generation, including `sphinx`
+ * Important environment variables
+   * User guide generation requires enviroment variables set by Read the Docs dashboard (`Admin` -> `Environment Variables`). Delete and add a new variable to modify
+   * Current environment variables set:
+     * `CC`: mpicc.mpich
+     * `PNETCDF_DIR`: /home/docs/checkouts/readthedocs.org/user_builds/pnetcdf-python/checkouts/latest/_readthedocs/PnetCDF
+     * `PNETCDF_VER`: 1.12.3