Add guidelines for contributing.

CONTRIBUTING.md

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+How to contribute to PMI
+========================
+
+(Note that PMI, as a submodule of IMP, follows the same contribution guidelines
+as IMP itself, below.)
+
+IMP is open source software, and we welcome contributions! These contributions
+could include
+ - Reporting or fixing a bug
+ - Fixing a typo in the comments or documentation
+ - Adding a new function, class, or module to IMP
+ - Publishing your protocol for applying IMP to a biological system
+
+To report a bug, please [open an issue at GitHub](https://github.com/salilab/pmi/issues).
+See [the IMP manual](https://integrativemodeling.org/nightly/doc/manual/bugs.html)
+for more information.
+
+The easiest way to fix a typo in the comments or documentation is to use
+the edit icon at the top of each page in GitHub.
+
+To add or change code in PMI, fork the repository and then open a pull request.
+Code should conform to our
+[coding conventions](https://integrativemodeling.org/nightly/doc/manual/code_conventions.html)
+and
+[naming and interface conventions](https://integrativemodeling.org/nightly/doc/manual/conventions.html).
+See the [IMP manual](https://integrativemodeling.org/nightly/doc/manual/developing.html)
+for more detail.
+
+To publish your protocol for applying IMP to a biological system, please
+see the [IMP manual](https://integrativemodeling.org/nightly/doc/manual/biosystem.html).