Update readme for spark version 1.10 as merge commit 67d067a206e has upgraded to build against 1.1.0

reference
https://github.com/amplab-extras/SparkR-pkg/commit/67d067a206e6bd583b07470345d0813a8253bf5e

* upgrade readme mentioning of spark 0.9.0
* removal oftext saying spark 09.0 is prerelease
* added links to devetools etc 
We shouldn't edit this line. SparkR is compatible with all Spark versions >= 0.9.0. We should just add a line that says that the default SparkR build using Spark 1.1.0