Updated README

Author: astrofrog

README.md

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+Integration testing for the astropy ecosystem
+=============================================
+
+This repository is a work in progress to develop a way to do integration testing
+across the astropy ecosystem to ensure that the core and coordinated packages
+work well together.
+
+For now, you can test the latest astropy release with the latest coordinated
+package releases (along with a few third-party packages that depend heavily
+on astropy). Pre-releases are used if available. To run the tests, run:
+
+    $ tox
+
+or you can even run all environments in parallel with e.g.:
+
+    $ tox --parallel 16