Add contribution guidelines

CONTRIBUTING.md

+# Contributing
+
+Interested in contributing to the Material theme? Before you do, please read
+the following guidelines.
+
+## Submission context
+
+### Got a question or problem?
+
+For quick questions there's no need to open an issue as you can reach us on
+[gitter.im][1].
+
+  [1]: https://gitter.im/mkdocs-material/general
+
+### Found a bug?
+
+If you found a bug in the source code, you can help us by submitting an issue
+to our GitHub Repository. Even better, you can submit a Pull Request with a
+fix. However, before doing so, read the submission guidelines.
+
+### Missing a feature?
+
+You can request a new feature by submitting an issue to our GitHub Repository.
+If you would like to implement a new feature, please submit an issue with a
+proposal for your work first, to be sure that it is of use for everyone, as
+the Material theme is highly opinionated. Please consider what kind of change
+it is:
+
+* For a **Major Feature**, first open an issue and outline your proposal so
+  that it can be discussed. This will also allow us to better coordinate our
+  efforts, prevent duplication of work, and help you to craft the change so
+  that it is successfully accepted into the project.
+
+* **Small Features** and bugs can be crafted and directly submitted as a Pull
+  Request. However, there is no guarantee that your feature will make it into
+  the master, as it's always a matter of opinion whether if benefits the
+  overall functionality of the theme.
+
+## Submission guidelines
+
+### Submitting an issue
+
+Before you submit an issue, please search the issue tracker, maybe an issue for
+your problem already exists and the discussion might inform you of workarounds
+readily available.
+
+We want to fix all the issues as soon as possible, but before fixing a bug we
+need to reproduce and confirm it. In order to reproduce bugs we will
+systematically ask you to provide a minimal reproduction scenario using the
+custom issue template.
+
+Unfortunately we are not able to investigate / fix bugs without a minimal
+reproduction, so if we don't hear back from you we are going to close issues
+that don't have enough information to be reproduced.
+
+### Submitting a Pull Request (PR)
+
+Search GitHub for an open or closed PR that relates to your submission. You
+don't want to duplicate effort. Fork the project, make your changes in **a new
+git branch** and commit your changes with a descriptive commit message. Then
+push your branch to GitHub and send a PR to `mkdocs-material:master`. If we
+suggest changes, make the required updates, rebase your branch and push
+the changes to your GitHub repository, which will automatiaclly update your PR.
+
+Afer your PR is merged, you can safely delete your branch and pull the changes
+from the main (upstream) repository.