[Jetsnack] Add README and screenshots.

Change-Id: I1d5746f11444a47caa1643a9862e689989a40412

Author: nickbutcher

Diff for: Jetsnack/README.md

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+# Jetsnack sample
+
+Jetsnack is a sample snack ordering app built with [Jetpack Compose][compose]. This sample showcases:
+
+* How to implement a custom design system
+* Custom layout
+* Animation
+
+<img src="screenshots/jetsnack.gif"/>
+
+### Status: 🚧 In progress 🚧
+
+Jetsnack is still under development and some screens are not yet implemented.
+
+## Features
+
+### Custom Design System
+
+Jetsnack's major feature is demonstrating how to implement a custom design system. Jetsnack has a bespoke color system and does not use [Material color theming](https://material.io/design/color/the-color-system.html).
+
+<img src="screenshots/color_system.png"/>
+
+This is implemented by:
+
+* [`JetsnackColorPalette`](app/src/main/java/com/example/jetsnack/ui/theme/Theme.kt#L114) a class modelling the desired color system.
+* [`JetsnackColorAmbient`](app/src/main/java/com/example/jetsnack/ui/theme/Theme.kt#L231) an [ambient](https://developer.android.com/reference/kotlin/androidx/compose/Ambient) holding the current color set.
+* [`ProvideJetsnackColors`](app/src/main/java/com/example/jetsnack/ui/theme/Theme.kt#L222) a composable function [providing](https://developer.android.com/reference/kotlin/androidx/compose/package-summary#Providers(androidx.compose.ProvidedValue,%20kotlin.Function0)) a `JetsnackColorPalette`
+* [`JetsnackTheme` object](app/src/main/java/com/example/jetsnack/ui/theme/Theme.kt#L104), providing convenient access to the current theme colors.
+* [`JetsnackTheme` composable](app/src/main/java/com/example/jetsnack/ui/theme/Theme.kt#L81), the app's theme. Note that while Jetsnack implements a custom color system, it still uses Material's shape and type theming.
+
+Jetsnack wraps Material components, customizing them to use its color system. See the [components package](app/src/main/java/com/example/jetsnack/ui/components) e.g. [`JetsnackButton`](app/src/main/java/com/example/jetsnack/ui/components/Button.kt). Jetsnack makes heavy use of gradients, see [`Gradient`](app/src/main/java/com/example/jetsnack/ui/components/Gradient.kt) for a number of helpful [`Modifier`](https://developer.android.com/reference/kotlin/androidx/compose/ui/Modifier)s.
+
+### Custom Layout
+
+<img src="screenshots/snack_details.gif"/>
+
+Jetsnack utilizes custom [`Layout`](https://developer.android.com/reference/kotlin/androidx/compose/ui/package-summary#layout_1)s to achieve its design. See:
+
+* [`CollapsingImageLayout`](app/src/main/java/com/example/jetsnack/ui/snackdetail/SnackDetail.kt#L274) shown above.
+* [`SearchCategory`](app/src/main/java/com/example/jetsnack/ui/home/search/Categories.kt#L97) custom positioning of an image and text items.
+* [`JetsnackBottomNavLayout`](app/src/main/java/com/example/jetsnack/ui/home/Home.kt#L170) a custom Bottom Navigation implementation which animates the width of selected/unselected items.
+
+## Data
+Domain types are modelled in the [model package](app/src/main/java/com/example/jetsnack/model), each containing static sample data exposed using fake `Repo`s objects.
+
+Imagery is sourced from [Unsplash](https://unsplash.com/) and loaded using [coil-accompanist][coil-accompanist].
+
+
+## License
+
+```
+Copyright 2020 The Android Open Source Project
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    https://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+```
+
+[compose]: https://developer.android.com/jetpack/compose
+[coil-accompanist]: https://github.com/chrisbanes/accompanist