Add decompiles and from tasty

Author: nicolasstucki

docs/blog/_posts/2017-11-21-fifth-dotty-milestone-release.md

@@ -28,6 +28,15 @@ support for Scala 2.12 and came with a brand new REPL.
 ### Reworked implicit search [#3421](https://github.com/lampepfl/dotty/pull/3421)
 TODO @odersky
 
+### TASTY frontend
+The TASTY frontend is a frontend that reloads the ASTs from the TASTY within the classfiles in the classpath. We fixed most issues with the frontend which enable us to re-compile code from TASTY without the need of retypecheck (#3436, #3512, #3492). 
+
+Currenlty two backends that use this frontend are available
+* Dotty class file decompiler `dotc -decompile -classpath CLASSPATH CLASSNAME`. It will decompile the code by preaty printing the contents of the TASTY. This code is the high level representation of the typechecked code without loss of precieceness due transformations towards JVM bytecode. It can also be used with `dotc -decompile -print-tasty ...` to show a preaty printed version of the raw TASTY (#3519).
+* Dotty TASTY compiler `dotc -from-tasty -classpath CLASSPATH CLASSNAME` will recompile code stored in the TASTY. This is the first step toward linking and whole word optimisations, recompiling code to different backends not present in the original compilation, recompiling code with different added plugins, .... In fact the decompiler is a TASTY compiler that compiles trees to strings.
+
+Known issues: #3490, #3500.
+
 
 ## Trying out Dotty
 ### Scastie