[Catalog] Add README.

Reviewers: #material_components_ios_owners, ajsecord, randallli, junius

Reviewed By: #material_components_ios_owners, ajsecord, randallli, junius

Projects: #material_components_ios

Differential Revision: http://codereview.cc/D123

Author: Jeff Verkoeyen

catalog/MDCCatalog.xcworkspace/contents.xcworkspacedata

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+# Catalog by convention
+
+This app is dynamically generated by taking advantage of CocoaPods wildcards. The Podfile for the
+catalog adds all Material components. It also adds any example source that matches a wildcard like
+`components/*/examples/*.{h,m,swift}`. Check out `material-components-ios-catalog.podspec` for the
+most up-to-date paths.
+
+At runtime, the app enumerates all instances of UIViewController that respond to the
+`+catalogHierarchy` class method. This method is expected to return an array of strings that define
+the "breadcrumbs" for the view controller in the app.
+
+The app uses the aggregate of these breadcrumbs to build a navigation tree, with each node either
+representing a specific UIViewController (an example) or a list of other nodes.
+
+## Building the app
+
+We use CocoaPods to manage the dependencies of this app. To generate the dependent projects, run
+the following within the `catalog/` directory of this repo:
+
+    pod install
+
+This will update the MDCCatalog.xcworkspace.
+
+Open the workspace and ensure that the "MDCCatalog" target is selected.
+
+Build and run the app.
+
+## Adding examples
+
+Let's build a hypothetical example for the "Ink" component.
+
+The first step is to create a source file. This can be either a .m or a .swift; whichever you
+prefer. Place this source file in the component's `examples/` directory, like so:
+
+> View controller names must be globally-unique across every component's example set. An easy way to
+> ensure this is to prefix the controller with the name of the component.
+
+    components/Ink/examples/InkDemoViewController.m
+
+Note that, like unit tests, you likely won't need to create a .h file for your example.
+
+You can now create the view controller class.
+
+    #import <UIKit/UIKit.h>
+
+    @interface InkDemoViewController : UIViewController
+    @end
+
+    @implementation InkDemoViewController
+
+    // TODO: Support other categorizational methods.
+    + (NSArray *)catalogHierarchy {
+      return @[ @"Ink", @"README demo" ];
+    }
+
+    @end
+
+The `catalogHierarchy` method is the mechanism by which you define *where* the example lives in the
+catalog app. It is easiest to think of the values as a list of breadcrumbs.
+
+To add the example to the Catalog app, simply run pod install in the catalog/ directory again:
+
+    catalog/ $ pod install
+
+Build and run and you'll see your example listed in the app's hierarchy.