Adding contribution document:

Author: Mark Gibbs

CONTRIBUTIONS.md

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+## Contributions
+
+Contributions, under the same MIT license as the repository, are most welcome.
+
+See the [development](https://django-plotly-dash.readthedocs.io/en/latest/development.html) section of
+the documentation for details on how to build, test and document contributions.
+
+Once ready, submit a pull request to the [main repository](https://github.com/GibbsConsulting/django-plotly-dash).
+
+Unless you request otherwise, you will be added to the list of contributors below.
+
+## Contributors
+
+Thanks to the following people who have submitted pull requests that have made it into the codebase
+
+[delsim](https://github.com/delsim)
+[weber-s](https://github.com/weber-s)