Update README.md
Added note about it being default on dev
Mycroft [![Build Status](https://travis-ci.org/MycroftAI/mycroft-core.svg?branch=master)](https://travis-ci.org/MycroftAI/mycroft-core)
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+NOTE: The default branch for this repo is dev. This should be considered a working beta. If you want to clone a more stable version, switch over to the 'master' branch.
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Full docs at: https://docs.mycroft.ai
Release notes at: https://docs.mycroft.ai/release-notes
