Extract contribution guidelines from the README

CONTRIBUTING.md

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+# Contribution guidelines #
+
+[![Chat on Gitter](https://badges.gitter.im/cristim/autospotting.svg)](https://gitter.im/cristim/autospotting?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
+
+The usual GitHub contribution model applies, but if you would like to [raise an
+issue](https://github.com/cristim/autospotting/issues/new) or start working on a
+[pull request](https://github.com/cristim/autospotting/pulls), please get in
+touch on [gitter](https://gitter.im/cristim/autospotting) to discuss it first to
+we make sure everything is clear and that nobody else is working on it.
+
+Any random questions are also better asked there.
+
+## Feature requests ##
+
+The use case should be presented in detail in the issue, and should also
+be discussed on gitter to make sure nothing was lost in translation.
+
+## Bug reports ##
+
+Bug reports should contain enough details to be reproduced by a developer.
+
+The commonly required information is already pre-filled when creating any GitHub
+issue, but be prepared to provide more when asked, either in the issue comments
+or on gitter.
+
+## Contribution standards ##
+
+The non-trivial code contributions need to provide unit tests for the
+functionality being changed.
+
+All the pull requests will need to pass a review by the maintainers before they
+can be merged. Be prepared to iterate your pull request until it is ready to be
+approved, which may take a while for bigger contributions.
+
+Pull requests should also have a corresponding issue linked to them so it is
+clear what problem they are solving.