Improve instructions in README

[sslavic]

Following README, with build.sbt as

org.eigengo.sbtraml.RamlPlugin.settings
name := "sbt-raml-example"
organization := "foo.bar"
scalaVersion := "2.10.4"
scalacOptions ++= Seq("-target:jvm-1.7", "-deprecation", "-unchecked", "-encoding", "utf8", "-feature", "-language:higherKinds", "-language:implicitConversions", "-language:postfixOps")
javacOptions in (Compile, compile) ++= Seq("-source", "1.7", "-target", "1.7")
source in Raml := baseDirectory.value / "src/raml"

project/build.properties as:

sbt.version=0.13.7

and project/plugins.sbt as:

resolvers += "sonatype-snapshots" at "https://oss.sonatype.org/content/repositories/snapshots"
resolvers += "Mulesoft" at "http://repository.mulesoft.org/releases"
addSbtPlugin("org.eigengo" % "sbt-raml" % "0.1-SNAPSHOT" changing())

sbt fails with error:

~/sbt-raml-example/build.sbt:13: error: not found: value source
source in Raml := baseDirectory.value / "src/some/custom/raml/path"

Since RamlPlugin is AutoPlugin having org.eigengo.sbtraml.RamlPlugin.settings in build.sbt isn't needed in sbt 0.13.5+.
Instead, to be able to configure RamlPlugin settings like source in Raml config, one needs to

import org.eigengo.sbtraml.RamlPlugin.Keys._

Consider documenting that.