Add release instruction file

Author: SylvainCorlay

RELEASE.md

@@ -0,0 +1,11 @@
+To release a new version of bqplot on PyPI:
+
+Update _version.py (set release version, remove 'dev')
+git add and git commit
+python setup.py sdist upload
+python setup.py bdist_wheel upload
+git tag -a X.X.X -m 'comment'
+Update _version.py (add 'dev' and increment minor)
+git add and git commit
+git push
+git push --tags