Add README

Author: julienrf

README.md

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+# Scala Collection Contrib
+
+This module provides extra features to the Scala standard collections.
+
+## New Collection Types
+
+- `MultiSet` (both mutable and immutable)
+- `SortedMultiSet` (both mutable and immutable)
+- `MultiDict` (both mutable and immutable)
+- `SortedMultiDict` (both mutable and immutable)
+
+## New Operations
+
+The new operations are provided via an implicit enrichment. You need to add the following
+import to make them available:
+
+~~~ scala
+import scala.collection.decorators._
+~~~
+
+The following operations are provided:
+
+- `Seq`
+    - `intersperse`
+- `Map`
+    - `zipByKey` / `join` / `zipByKeyWith`
+    - `mergeByKey` / `fullOuterJoin` / `mergeByKeyWith` / `leftOuterJoin` / `rightOuterJoin`
+