Add to drop features documentation

Author: odersky

docs/docs/reference/dropped-features/symlits.md

@@ -0,0 +1,7 @@
+---
+layout: doc-page
+title: Dropped: Symbol Literals
+---
+
+Symbol literals are no longer supported. The `scala.Symbol` class still exists, so a
+literal translation of the symbol literal `'xyz` is `Symbol("xyz")`. However, it is recommended to use a plain string literal `"xyz"` instead. (The `Symbol` class will be deprecated and removed in the future).

docs/sidebar.yml

@@ -109,6 +109,8 @@ sidebar:
               url: docs/reference/dropped-features/limit22.html
             - title: XML literals
               url: docs/reference/dropped-features/xml.html
+            - title: Symbol Literals
+              url: docs/reference/dropped-features/symlits.html
             - title: Auto-Application
               url: docs/reference/dropped-features/auto-apply.html
             - title: Weak Conformance