First cut at CONTRIBUTING

CONTRIBUTING.md

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+
+## Bug reports
+
+We welcome and encourage bug reports for the Rubber Band
+Library. We're happy to receive them by almost any channel. The most
+usual are [as Github issues](https://github.com/breakfastquay/rubberband/issues)
+or [by contacting us directly](https://breakfastquay.com/contact.html).
+
+Please try to provide enough information for us to reproduce the
+problem. At the minimum this includes details of platform, compiler,
+and library configuration, as well as a description of the expected
+and problematic behaviour. We appreciate that describing issues
+relating to audio output can be tricky, and we may ask you to send or
+provide further information such as audio examples.
+
+## Pull requests
+
+Github pull requests may be appropriate if you have small fixes, such
+as fixes to the build system or documentation, that would be
+convenient to upstream. We will gratefully read and consider PRs and
+consider applying changes of this sort.
+
+However, please be aware that we cannot directly merge pull
+requests. This is partly for technical reasons (the Github repository
+for the library is a read-only mirror of an upstream repo) and partly
+for reasons of copyright and relicensing.
+
+If you have any more substantial changes you would like to see
+included, please contact us directly to discuss.
+
+## Other ways to contribute
+
+An excellent way to contribute to the continued development of Rubber
+Band Library is to [buy a commercial licence](https://breakfastquay.com/technology/license.html).
+Or, for enhancements and other new development work, it may be possible to
+engage [Particular Programs Ltd](https://particularprograms.co.uk/) at
+a commercial rate. Please contact us for details.
+