Add links to official docs on trees and symbols

Author: Pere Villega

src/main/tut/docs/symbols.md

@@ -3,8 +3,6 @@ layout: docs
 title: Symbols
 ---
 
-Symbols in scalac
-
-WIP
+The official documentation of the compiler has a great section explaining [Symbols](http://docs.scala-lang.org/overviews/reflection/symbols-trees-types.html).
 
 Want to contribute? [Edit this file](https://github.com/typelevel/scala/edit/typelevel-readme/src/main/resources/microsite/docs/symbols.md)

src/main/tut/docs/trees.md

@@ -6,12 +6,8 @@ title: Trees
 This page describes the Scala [AST]() and its implementation within the Scala compiler code. 
 
 The AST is designed to capture the syntactic constructs of the Scala Programming Language, 
-as described in the [language specification](https://www.scala-lang.org/files/archive/spec/2.12/13-syntax-summary.html).
+as described in the [language specification](https://www.scala-lang.org/files/archive/spec/2.12/13-syntax-summary.html). The implementation of the Tree is encoded in [this file](https://github.com/scala/scala/blob/2.13.x/src/reflect/scala/reflect/internal/Trees.scala) 
 
-The implementation of the Tree is encoded in [this file](https://github.com/scala/scala/blob/2.13.x/src/reflect/scala/reflect/internal/Trees.scala) 
-
-The AST is organised in a series of traits and self-types, which follows the following structure: 
-
-WIP
+The official documentation of the compiler has a great section giving more details on  [Trees](http://docs.scala-lang.org/overviews/reflection/symbols-trees-types.html)/
 
 Want to contribute? [Edit this file](https://github.com/typelevel/scala/edit/typelevel-readme/src/main/resources/microsite/docs/trees.md)