Documenting release procedure

RELEASE.md

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+# Release procedure for shub
+
+The Travis build is configured to release shub to PyPI whenever
+a new tag is committed.
+
+The steps to do a release are:
+
+1) Install [bumpversion](https://pypi.python.org/pypi/bumpversion)
+
+2) Make sure you're in the tip of master, and then run:
+
+    bumpversion VERSION_PART
+
+In place of VERSION_PART, use one of `patch`, `minor` or `major`, meaning the
+part of the version number to be updated.
+
+This will create a new commit and tag updating the version number.
+
+3) Push the changes and the new tag to trigger the release:
+
+    git push && git push --tags
+
+4) Once the build finishes, run `pip install shub` in a temporary virtualenv
+and make sure it's installing the latest version.
+
+5) Update the release information in:
+
+https://github.com/scrapinghub/shub/releases