Add very basic flutter.yaml documentation.

flutter_yaml.md

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+---
+layout: page
+title: flutter.yaml Format
+permalink: /manifest/
+---
+
+Every Flutter application needs some metadata so it can specify its dependencies
+(services, assets, etc.). Flutter stores this information in a file named
+`flutter.yaml`, which is written in the [YAML](http://www.yaml.org/) language.
+
+At the top level are a series of fields. The currently supported ones are:
+
+`name`
+: Required for every application.
+
+`assets`
+: Optional. Array of strings. Relative paths to assets for the application.
+
+`material-design-icons`
+: Optional. Array of dictionaries specifying icons to include as assets.
+
+`services`
+: Optional. Array of strings.  Names of platform and third-party services
+(Firebase, Google Cloud Messaging, etc.) to expose to your application.
+
+`version`
+: Optional. String used by [updater service][]. Follows [semantic versioning][].
+
+`update-url`
+: Optional. Url string used by [updater service][].
+
+All other fields are ignored. A complete `flutter.yaml` looks something like the
+following:
+
+{% prettify yaml %}
+name: example
+assets:
+  - assets/flutter_logo.png
+  - packages/flutter_gallery_assets/sun.png
+material-design-icons:
+  - name: action/alarm
+  - name: action/android
+services:
+  - firebase
+{% endprettify %}
+
+## Name
+
+Every application needs a name.  This is used by the flutter tool when
+manipulating files relating to your application on your behalf.
+
+There are no restrictions at this time on valid characters in a name.
+
+## Assets
+
+Assets are optional, however nearly every application includes them.  These are
+any files which you wish to have bundled in your applications `flx` file for
+access via the `asset_bundle` service, or through the `AssetImage` widget, etc.
+
+## Material Design Icons
+
+Material Design Icons are optional. These icons are separate from general assets
+as the flutter tools know how to handle bundling the right icons for the devices
+you are targeting.
+
+Google [documents](https://design.google.com/icons/) the full list of available
+material design icons.  If you find one missing from Flutter, please [file an
+issue](https://github.com/flutter/flutter/issues/new)
+
+## Services
+
+Services are optional, but most applications will want to include them.
+Services are how Flutter applications access platform APIs or third party
+frameworks.
+
+At time of writing only two services are optional `firebase` and `gcm`.
+
+## Version
+
+A version number is only required to update your application via the
+[updater service][].
+
+Each time you publish your package, you publish it at a specific version.
+Once that's been done, consider it hermetically sealed: you can't touch it
+anymore. To make more changes, you'll need a new version.
+
+When you select a version, follow [semantic versioning][].
+
+# Update URL
+
+An update url is only required to update your application via the
+[updater service][].
+
+
+[semantic versioning]: http://semver.org/spec/v2.0.0.html
+[updater service]: https://github.com/flutter/flutter/wiki/On-the-fly-app-updates