Merge pull request #1144 from rcurtin/contributing

Add guidelines and process for contributing.

CONTRIBUTING.md

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+# Contributing to mlpack
+
+Anyone is welcome to contribute to mlpack and join the community.  If you would
+like to make improvements to the library or have found a bug that you know how
+to fix, please submit a pull request!
+
+If you would like to learn more about how to get started contributing, see
+[Getting Involved](http://www.mlpack.org/involved.html), and if you are
+interested in participating in Google Summer of Code, see
+[mlpack and Google Summer of Code](http://www.mlpack.org/gsoc.html).
+
+## Pull request process
+
+Once a pull request is submitted, it must be reviewed by at least one member of
+mlpack's Contributors team, to ensure that (if applicable):
+
+ * the design meshes with the rest of mlpack
+ * the style matches the
+   [Style Guide](http://github.com/mlpack/mlpack/wiki/DesignGuidelines)
+ * any new functionality is tested and working
+
+Once the pull request is approved by one member of the Contributors team, it may
+be merged between 3 and 7 days after approval.  This allows other contributors
+and maintainers to have time to also review the PR.  If a pull request has at
+least two approvals from members of the Contributors team, then the pull
+request may be immediately merged.  This applies even if the submitter of the
+PR is a member of the Contributors team.