Add implementation for the EarlyLintPass trait into the Adding Lints documentation.

To follow along with the adding lint documentation you need to implement this trait in order to register the lint pass.

Author: markcatley

doc/adding_lints.md

@@ -115,7 +115,7 @@ where all the lint code is. We are going to call the file
 `clippy_lints/src/foo_functions.rs` and import some initial things we need:
 
 ```rust
-use rustc::lint::{LintArray, LintPass};
+use rustc::lint::{LintArray, LintPass, EarlyLintPass};
 use rustc::{declare_tool_lint, lint_array};
 ```
 
@@ -161,6 +161,8 @@ impl LintPass for FooFunctionsPass {
         "FooFunctions"
     }
 }
+
+impl EarlyLintPass for FooFunctionsPass {}
 ```
 
 Don't worry about the `name` method here. As long as it includes the name of the