Added Code of Conduct that points to TSC (#4124)

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<!--- - [ ] My code follows the code style of this project. -->

- [ ] I have read and agree to abide by the [Code of Conduct](https://usgs-astrogeology.github.io/code/)

- [ ] I have read and agree to abide by the [Code of Conduct](https://github.com/USGS-Astrogeology/ISIS3/blob/dev/Code-Of-Conduct.md)

- [ ] I have read the [**CONTRIBUTING**](https://github.com/USGS-Astrogeology/ISIS3/blob/dev/CONTRIBUTING.md) document.

- [ ] My change requires a change to the documentation.

- [ ] I have updated the documentation accordingly.

# Code of Conduct

This project uses the Planetary Software Code of Conduct which can be found at https://github.com/planetarysoftware/TSC/blob/master/Code-Of-Conduct.md

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