add CRAN badge

Author: nsheff

README.md

@@ -1,7 +1,7 @@
 simpleCache: R caching for restartable analysis
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-<a href="https://travis-ci.org/databio/simpleCache"><img src="https://travis-ci.org/databio/simpleCache.svg?branch=master" alt="Travis CI status"></img></a>
+<a href="https://travis-ci.org/databio/simpleCache"><img src="https://travis-ci.org/databio/simpleCache.svg?branch=master" alt="Travis CI status"></img></a> <img src="https://www.r-pkg.org/badges/version/simpleCache"></img>
 
 `simpleCache` is an R package providing functions for caching R objects. Its
 purpose is to encourage writing reusable, restartable, and reproducible analysis