Merge pull request #456 from dvirf1/master

adding a friendly message when the implicit for Derivation is not found

macros/src/main/scala/pureconfig/Derivation.scala

@@ -1,11 +1,14 @@
 package pureconfig
 
+import scala.annotation.implicitNotFound
 import scala.collection.mutable
 import scala.language.experimental.macros
 import scala.reflect.macros.whitebox
 
 import pureconfig.derivation._
 
+@implicitNotFound("""Cannot find an implicit instance of ${A}.
+If you are trying to read or write a case class or sealed trait consider using PureConfig's auto derivation by adding `import pureconfig.generic.auto._`""")
 sealed trait Derivation[A] {
   def value: A
 }