Add the version table

The table shows the compatible RxJava version for RxScala.
Here. I mean people can specify other RxJava version in their pom.xml, build.gradle, or build.sbt, e.g.,

```
libraryDependencies ++= Seq(
  "io.reactivex" % "rxjava" % "1.0.3",
  "io.reactivex" %% "rxscala" % "0.23.0")
```

in case RxScala doesn't release a version for RxJava 1.0.3 in time, but they want to update RxJava to 1.0.3 for some critical bug fixes.