Create CONTRIBUTING.md

Author: svgeesus

CONTRIBUTING.md

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+# How to contribute
+
+This repository containes the current Web MIDI API _standard document_, developed by the Audio Working Group at the W3C. 
+There are _multiple implementations_ of the Web MIDI API standard.
+
+Feature requests are welcome, but a quick search in the [opened and closed issues](https://github.com/WebAudio/web-midi-api/issues) 
+is welcome to avoid duplicates.
+
+Pull requests are also welcome, but any change to the standard (barring things like typos and the like) will have 
+to be discussed in an issue (and possibly during a call, accessible only to W3C Audio Working Group members).