Add info about CodeMap for rust-lang#50

Author: vishalsodani

src/code-index.md

@@ -8,3 +8,4 @@ Item            |  Kind    | Short description           | Chapter            |
 ----------------|----------|-----------------------------|--------------------|-------------------
 `TyCtxt<'cx, 'tcx, 'tcx>` | type | The "typing context". This is the central data structure in the compiler. It is the context that you use to perform all manner of queries. | [The `ty` modules](ty.html) | [src/librustc/ty/context.rs](https://github.com/rust-lang/rust/blob/master/src/librustc/ty/context.rs)
 `ParseSess` | struct | This struct contains information about a parsing session | [The parser](the-parser.html) | [src/libsyntax/parse/mod.rs](https://github.com/rust-lang/rust/blob/master/src/libsyntax/parse/mod.rs)
+`CodeMap` | struct | The CodeMap maps the AST nodes to their source code | [The parser](the-parser.html) | [src/libsyntax/codemap.rs](https://github.com/rust-lang/rust/blob/master/src/libsyntax/codemap.rs)