Adds to the contributor docs about transforms

CONTRIBUTING.md

@@ -128,6 +128,69 @@ to read on GitHub as well as in several git tools.
 For more information about what each part of the template mean, head up to the documentation in the
 [angular repo](https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format)
 
+## --migrate with the CLI
+
+This is a new feature in development for the CLI.
+
+```
+webpack --migrate <your-config-name>
+```
+
+The expected result of the above command is to take the mentioned `webpack` configuration and create a new configuration file which is compatible with webpack 2.
+It should be a valid new config and should keep intact all the features from the original config.
+The new config will be as readable as possible (may be add some comments).
+
+With [#40](https://github.com/webpack/webpack-cli/pull/40), we have been able to add basic scaffolding and do many of the conversions recommended in the [docs](https://webpack.js.org/guides/migrating/).
+
+### How it's being done
+
+We use [`jscodeshift`](https://github.com/facebook/jscodeshift) transforms called `codemods` to accomplish this.
+We have written a bunch of transformations under [/lib/transformations](https://github.com/webpack/webpack-cli/tree/master/lib/transformations),divided logically.
+We convert the existing webpack config to [AST](https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Parser_API). We then parse this tree for the specific features and modify it to conform to webpack v2...
+
+#### Structure of a transform
+
+The directory structure of a transform looks as follows -
+
+```sh
+|
+|--__snapshots__
+|--__testfixtures__
+|  |
+|  |--transform-name.input.js
+|
+|--transform-name.js
+|--transform-name.test.js
+```
+
+`transform-name.js`
+
+This file contains the actual transformation codemod. It applies specific transformation and parsing logic to accomplish its job
+There are utilities available under `/lib/utils.js` which can help you with this.
+
+`transform-name.test.js`
+
+This is where you declare a new test case for your transformation.
+Each test will refer to an input webpack config snippet.
+Conventionally we write them in `\_\_testfixtures\_\_`.
+
+```
+const defineTest = require('../defineTest');
+
+defineTest(__dirname, 'transform-name.input1.js');
+defineTest(__dirname, 'transform-name.input2.js');
+```
+
+`defineTest` is a helper test method which helps us to run tests on all the transforms uniformly.
+It takes the input file given as parameter and uses jest to create a snapshot of the output. This effectively tests the correctness of our transformation.
+
+### TODO
+
+This is still in a very raw form. We'd like to take this as close to a truly useful tool as possible.
+We will still need to
+  - Support all kinds of webpack configuration(made using merge tools)
+  - Test these transforms against real world configurations.
+
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