Describe relationship between flatland/useful and amalloy/useful repos

Author: Andy Fingerhut

README.md

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 [![Build Status](https://secure.travis-ci.org/flatland/useful.png)](http://travis-ci.org/flatland/useful)
+
+These two repositories are usually identical:
+
+* https://github.com/amalloy/useful
+* https://github.com/flatland/useful
+
+The first is the canonical one.  Fixes tend to appear there first, and
+that is the preferred repository for filing issues on Github.  Fixes
+are backported to the second one periodically.