Added contributing info from legacy repo

Author: spcarte

CONTRIBUTING.md

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+# Contributing
+Any community support for the development of ForceFinder is welcomed. This support could come in many forms:
+
+- Code development for inverse methods
+- Code development for error metrics
+- Code development for unit and integration tests
+- Using the package and reporting bugs
+- Using the package and reporting workflow pain points
+
+Additional development dependencies may be installed for those that are supporting code development, which makes it easy to run the tests and build the documentation. These extra dependencies can be installed with one of two methods:
+
+- pip install -e .[dev]
+- pip install -r requirements-dev.txt 

requirements-dev.txt

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+pytest
+pytest-cov
+jupyter-book
+sphinx-autoapi