Add scalafix configuration

it fails because of a mis-configuration of the ExplicitReturnTypes rewrite
error: Failed to fix /home/gui/scaladex/scaladex/server/src/main/scala/ch.epfl.scala.index.server/routes/api/PublishApi.scala94.4% [######### ] 17 source files fixed
java.lang.IllegalStateException: Object {} has no field 'explicitReturnTypes'. Did you mean '' instead?
        at metaconfig.Configured.get(Configured.scala:11)
        at scalafix.rewrite.ExplicitReturnTypes.scalafix$rewrite$ExplicitReturnTypes$$isRewriteCandidate$1(ExplicitReturnTypes.scala:95)
        at scalafix.rewrite.ExplicitReturnTypes$$anonfun$rewrite$8.isDefinedAt(ExplicitReturnTypes.scala:118)
        at scalafix.rewrite.ExplicitReturnTypes$$anonfun$rewrite$8.isDefinedAt(ExplicitReturnTypes.scala:116)
        at scala.meta.transversers.Api$XtensionCollectionLikeUI$traverser$4$.apply(Api.scala:30)
        at scala.meta.transversers.Traverser.apply(Traverser.scala:4)
        at scala.meta.transversers.Traverser.apply(Traverser.scala:4)
        at scala.meta.transversers.Api$XtensionCollectionLikeUI$traverser$4$.apply(Api.scala:31)
        at scala.meta.transversers.Traverser.apply(Traverser.scala:4)
        at scala.meta.transversers.Api$XtensionCollectionLikeUI$traverser$4$.apply(Api.scala:31)
        at scala.meta.transversers.Traverser.apply(Traverser.scala:4)
        at scala.meta.transversers.Traverser.apply(Traverser.scala:4)
        at scala.meta.transversers.Api$XtensionCollectionLikeUI$traverser$4$.apply(Api.scala:31)
        at scala.meta.transversers.Traverser.apply(Traverser.scala:4)
        at scala.meta.transversers.Traverser.apply(Traverser.scala:4)
        at scala.meta.transversers.Api$XtensionCollectionLikeUI$traverser$4$.apply(Api.scala:31)
        at scala.meta.transversers.Traverser.apply(Traverser.scala:4)
        at scala.meta.transversers.Traverser.apply(Traverser.scala:4)
        at scala.meta.transversers.Api$XtensionCollectionLikeUI$traverser$4$.apply(Api.scala:31)
        at scala.meta.transversers.Traverser.apply(Traverser.scala:4)

.scalafix.conf

@@ -0,0 +1,11 @@
+rewrites = [
+  ExplicitReturnTypes
+  RemoveUnusedImports
+  ProcedureSyntax
+  ExplicitUnit
+  NoAutoTupling
+]
+
+explicitReturnTypes {
+
+}

CONTRIBUTING.md

@@ -43,6 +43,10 @@ Make shure to run `bin/scalafmt` to format your code.
 
 You can intall a pre-commit hook with `bin/hooks.sh`
 
+## Scalafix
+
+run `sbt scalafix`
+
 ### Elasticsearch Remote
 
 If you have an elasticsearch service installed use the following sbt command when
@@ -215,5 +219,4 @@ github test user:
 user: foobarbuz 
 pass: tLA4FN9O5jmPSnl/LDkSb0cYgJDe8HHdOMkY2yZO4m0=
 api:  c61e65b80662c064abe923a407b936894b29fb55
-repo: git@github.com:foobarbuz/example.git
-
+repo: git@github.com:foobarbuz/example.git

project/plugins.sbt

@@ -4,5 +4,6 @@ addSbtPlugin("com.typesafe.sbt" % "sbt-twirl" % "1.3.2")
 addSbtPlugin("org.madoushi.sbt" % "sbt-sass" % "0.9.3")
 addSbtPlugin("com.typesafe.sbt" % "sbt-native-packager" % "1.1.1")
 addSbtPlugin("org.scala-js" % "sbt-scalajs" % "0.6.17")
+addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.5.0-M1")
 
 libraryDependencies += "com.typesafe" % "config" % "1.3.1"