Add CONTRIBUTING.md to try to redirect new issues/pull requests to sbt/sbt
Fix header syntax in CONTRIBUTING and update README with similar contents
use CrossVersion.full for unstable Scala by default

This partially rolls back
harrah/xsbt@13e62fd6452974a77eb9b9ac9a60a613316b1d14.
adds apiVersion and isAPICompatible.

This is an improved API compatibility check that attempts to parse the
version structure instead of simply checking for "-".
add sbtApiVersion, and rename apiVersion to scalaApiVersion
change binaryScalaVersion and binarySbtVersion to be aware of *ApiVersion