Prepared a basic contribution_guide for gprMax.

CONTRIBUTING_GUIDE.md

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+# Contributing to gprMax
+
+Thank you for your interest in contributing to gprMax, we really appreciate your time and effort!
+
+## How can you help us?
+
+* Report a bug
+* Improve documentation
+* Submit a bug fix
+* Propose new features
+* Discuss the code implementation
+* Test our latest version which is available through the [devel branch](https://github.com/gprmax/gprMax/tree/devel) on our repository.
+
+## How to Contribute
+
+In general, we follow the "fork-and-pull" Git workflow.
+
+1. Fork the gprMax repository
+2. Clone the repository
+3. Make the changes you want to make
+4. Run the following command before you commit your changes to ensure that your code is formatted correctly:
+
+```
+pre-commit run --all-files
+```
+
+5. Commit your changes locally as you progress
+6. Push your local branch to your fork
+7. Submit a Pull request so that we can review your changes
+
+> NOTE: Be sure to merge the latest from "upstream" before making a pull request!
+
+## Feature and Bug reports
+
+We use GitHub issues to track bugs and features. Report them by opening a [new issue](https://github.com/gprMax/gprMax/issues).
+
+## Code review process
+
+The Pull Request reviews are done frequently. Try to explain your PR as much as possible using our template. Also, please make sure you respond to our feedback/questions about the PR.
+
+## Community
+
+Please use our [Google Group](https://groups.google.com/g/gprmax) (Forum) for comments, interaction with other users, chat, and general discussion on gprMax, GPR, and FDTD.
+
+Checkout our website [gprmax.com](https://www.gprmax.com/) for more information and updates.