Add CONTRIBUTING documentation

CONTRIBUTING.md

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+The [Earth System Modeling Framework](https://earthsystemmodeling.org/) (ESMF)
+is high-performance, flexible software infrastructure for building and coupling
+weather, climate, and related Earth science applications. Please review the
+following guidelines for contributing to the Earth System Modeling Framework.
+
+[fork]: https://github.com/esmf-org/esmf/fork
+[pr]: https://github.com/esmf-org/esmf/compare
+[actions]: https://github.com/esmf-org/esmf/actions
+
+## Resources
+
+* ESMF User Guide - [html](https://earthsystemmodeling.org/docs/nightly/develop/ESMF_usrdoc), [pdf](https://earthsystemmodeling.org/docs/nightly/develop/ESMF_usrdoc.pdf)
+* Fortran Reference Manual - [html](https://earthsystemmodeling.org/docs/nightly/develop/ESMF_refdoc), [pdf](https://earthsystemmodeling.org/docs/nightly/develop/ESMF_refdoc.pdf)
+* C Reference Manual - [html](https://earthsystemmodeling.org/docs/nightly/develop/ESMC_crefdoc), [pdf](https://earthsystemmodeling.org/docs/nightly/develop/ESMC_crefdoc.pdf)
+* NUOPC Reference Manual - [html](https://earthsystemmodeling.org/docs/nightly/develop/NUOPC_refdoc), [pdf](https://earthsystemmodeling.org/docs/nightly/develop/NUOPC_refdoc.pdf)
+* ESMPy Documentation - [html](https://earthsystemmodeling.org/esmpy_doc/nightly/develop/html), [pdf](https://earthsystemmodeling.org/esmpy_doc/nightly/develop/ESMPy.pdf)
+* Developer’s Guide - [html](https://earthsystemmodeling.org/docs/nightly/develop/dev_guide)
+
+## Submitting a pull request
+
+1. [Fork][fork] and clone the repository
+2. Create a new branch: `git checkout -b my-branch-name`
+3. Make and [test](#testing) your changes
+4. Update [documentation](#documentation)
+5. Push to your fork
+6. Execute `ESMF CI` and `ESMF Docs` from your fork on your branch using [actions][actions]
+7. Compare across forks and [submit a pull request][pr]
+
+## Testing
+
+Pull requests must pass the [ESMF CI][actions] action before they are accepted
+into the `develop` branch. The [ESMF CI][actions] action is configured to test
+ESMF on a variety of configurations. These include multiple operating systems,
+MPI libraries, compilers, and build options. Prior to executing the
+[ESMF CI][actions] action, changes should pass `make all_tests` on a local
+machine or HPC. 
+
+## Documentation
+
+ESMF uses LaTeX and LaTeX extensions to build inline documentation.
+Documentation can be built using the [ESMF Docs][actions] action, which provides
+all the software needed to build documentation. Documentation can also be built
+locally using `texlive-full` and `latex2html`. ESMPY also requires the following
+Python packages: `sphinx`, `sphinxcontrib-packages` and `sphinxcontrib-bibtex`.
+
+* ESMF Documentation `cd ${ESMF_DIR} && make doc`
+* ESMF Developers Guide: `cd ${ESMF_DIR}/src/doc/dev_guide && make`
+* NUOPC Documentation `cd ${ESMF_DIR}/src/addon/NUOPC/doc && make localdoc`
+* ESMPY Documentation `cd ${ESMF_DIR}/src/addon/esmpy && . ${PYTHON_VENV_ACTIVATE} && python3 -m pip install . && cd doc && . ${PYTHON_VENV_ACTIVATE} && make html latexpdf`
+
+## Contact Us
+
+[ESMF Discussions](https://github.com/orgs/esmf-org/discussions) is a GitHub
+Discussions forum for publicly discussing ESMF concerns and requests. For more
+information please see the
+[ESMF Discussions How-To Guide](https://github.com/orgs/esmf-org/discussions/246).
+
+If you do not have a GitHub Account, if you have privacy concerns posting to
+ESMF Discussions, or if you prefer communication via email then please contact
+esmf_support@ucar.edu.
+
+---
+Thank You,<br> 
+The ESMF Core Team