Add lintrunner instructions to contributing guide (pytorch#397)

Summary:
Pull Request resolved: pytorch#397

tsia

Reviewed By: colin2328

Differential Revision: D36908437

fbshipit-source-id: e5859b4bc2c13c28f230463051b6ccd58920e01a

CONTRIBUTING.md

@@ -9,7 +9,24 @@ We actively welcome your pull requests.
 2. If you've added code that should be tested, add tests.
 3. If you've changed APIs, update the documentation.
 4. Ensure the test suite passes.
-5. Make sure your code lints.
+5. Make sure your code lints. You can use [lintrunner](https://github.com/pytorch/pytorch/wiki/lintrunner) to do so.
+ 1. To set up:
+    ```
+    pip install lintrunner
+    lintrunner init
+    ```
+ 2. To lint your local changes:
+    ```
+    lintrunner
+    ```
+ 3. To format locally changed files:
+    ```
+    lintrunner f
+    ```
+  4. To lint all files:
+    ```
+    lintrunner --all-files
+    ```
 6. If you haven't already, complete the Contributor License Agreement ("CLA").
 
 ## Contributor License Agreement ("CLA")