Set version to 0.4.0 and update README accordingly

Author: julienrf

CONTRIBUTING.md

@@ -18,12 +18,15 @@ You need the following:
 - You should perform your work in its own Git branch.
 - Then open a pull request on GitHub, with `master` as base branch.
 
+Have a look at the [Waffle.io board](https://waffle.io/scala/collection-strawman) to
+quickly know which issues are ready and which are already in progress.
+
 ### Build
 
 - Compile the collections and run the tests:
 
   ~~~
-  >; compile; test
+  >; compile; test; junit/test
   ~~~
 - Run the memory benchmark:
 

README.md

@@ -1,6 +1,7 @@
 # Collection-Strawman
 
 [![Join the chat at https://gitter.im/scala/collection-strawman](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/scala/collection-strawman?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Waffle.io board](https://badge.waffle.io/scala/collection-strawman.svg?label=ready&title=Ready issues)](http://waffle.io/scala/collection-strawman)
 
 Prototype improvements for Scala collections.
 
@@ -25,18 +26,18 @@ are available. If you see something missing, please
 Add the following dependency to your project:
 
 ~~~ scala
-libraryDependencies += "ch.epfl.scala" %% "collection-strawman" % "0.3.0"
+libraryDependencies += "ch.epfl.scala" %% "collection-strawman" % "0.4.0"
 ~~~
 
-The 0.3.0 version is compatible with Scala 2.13 and Dotty 0.2. Scala 2.12 is also supported
+The 0.4.0 version is compatible with Scala 2.13 and Dotty 0.2. Scala 2.12 is also supported
 but you might encounter type inference issues with it.
 
 We also automatically publish snapshots on Sonatype:
 
 ~~~ scala
 resolvers += Resolver.sonatypeRepo("snapshots")
 
-libraryDependencies += "ch.epfl.scala" %% "collection-strawman" % "0.4.0-SNAPSHOT"
+libraryDependencies += "ch.epfl.scala" %% "collection-strawman" % "0.5.0-SNAPSHOT"
 ~~~
 
 ### Migrating from the standard collections to the strawman

build.sbt

@@ -4,7 +4,7 @@ dotty in ThisBuild := "0.3.0-RC1"
 
 val commonSettings = Seq(
   organization := "ch.epfl.scala",
-  version := "0.4.0-SNAPSHOT",
+  version := "0.4.0",
   scalaVersion := "2.12.3",
   crossScalaVersions := scalaVersion.value :: "2.13.0-M2" :: dotty.value :: Nil,
   scalacOptions ++= Seq("-deprecation", "-feature", "-unchecked", "-language:higherKinds"/*, "-opt:l:classpath"*/),