Create experimental-backend.md

Author: lrytz

experimental-backend.md

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+# Experimental Backend in Scala 2.11
+
+Scala 2.11 ships with an experimental backend named "GenBCode", originally implemented by [Miguel Garcia](https://github.com/magarciaEPFL).
+This backend will become the new default in Scala 2.12.
+Given that the new backend is experimental in Scala 2.11, it can be modified and extended without compatibility concerns.
+Most of the work towards the Scala 2.12 backend happens in the 2.11 branch and flows into 2.11.x minor releases.
+
+This gives you the chance to preview pieces of Scala 2.12 just by enabling some compiler flags in 2.11!
+
+## Enabling GenBCode
+
+`-Ybackend:GenBCode` enables the new backend.
+
+GenBCode without any further compiler flags is a drop-in replacement for the current backend ("GenASM").
+The generated bytecode is binary compatible, altough not exactly the same; some instruction sequences may differ.
+If you find a binary incompatibility, please [report an issue](https://issues.scala-lang.org/secure/Dashboard.jspa).
+
+
+
+## Delambdafy-Method
+
+