DOC: Add CONTRIBUTING.md (#1549)

CONTRIBUTING.md

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+# Contributing to ANTs
+
+Contributions to ANTs are welcome. The developers are grateful to the many contributors to ANTs, both those who have [contributed to the ANTs source code](https://github.com/ANTsX/ANTs/graphs/contributors) and those who have contributed documentation, testing, feedback and ideas.
+
+## Contributing code
+
+### 1. Plan contribution with ANTs team
+
+Before doing a lot of work, we recommend that you open an issue or start a discussion thread to discuss your ideas. If fixing a bug, please identify the bug in an issue first, following the bug report template.
+
+### 2. Make changes and test locally
+
+Once your code compiles without error and runs satisfactorily, you can open a pull request. You can mention issues by their number, eg `See issue #1234`. If you're confident that you're completing an open issue, you can say `Fixes #1234`, this will close the associated issue when the PR is merged.
+
+### 3. Open a pull request
+
+All contributions must be made via pull request on a fork of the ANTs repository. The developers will review and give feedback. 
+
+Write your commit messages using the standard prefixes for ITK commit messages:
+
+* BUG: Fix for runtime crash or incorrect result
+* COMP: Compiler error or warning fix
+* DOC: Documentation change
+* ENH: New functionality
+* PERF: Performance improvement
+* STYLE: No logic impact (indentation, comments)
+* WIP: Work In Progress not ready for merge (used before PR is ready, or if a draft PR needs feedback or assistance)
+
+A Github continuous integration test will be run on the PR, confirming that it compiles on Linux and doesn't break basic functionality. This is a minimal test, please test new functionality and if at all possible do so on open data, so that the developers can reproduce results.
+
+
+### Licensing of contributions
+
+All accepted contributions will be incorporated into the ANTs source code and distributed according to the terms of the ANTs license.