Address the 'How do I just use it' FAQ (#246)

Adds documentation targetted at someone who has found the project hoping
to be able to run a command or two and have desktop support in their
existing Flutter application. Attempts to set expectations, but also
provide some more information about how someone could potentially get
started even without experience in desktop development.

Author: stuartmorgan

Flutter-Requirements.md

@@ -62,3 +62,13 @@ on all platforms.
 
 Symptoms of missing fonts can include text failing to display, console logging
 about failure to load fonts, or in some cases crashes.
+
+## Plugins
+
+If your project uses any plugins with platform components, they won't
+work, as the native side will be missing. Depending on how the Dart side of the
+plugin is written, they may fail gracefully, or may throw errors.
+
+You may need to make the calls to those plugins conditional based on the host
+platform. Alternately, if you have the expertise, you could implement the native
+side of the plugin in your desktop project(s).

Quick-Start.md

@@ -0,0 +1,48 @@
+# Quick Start
+
+A common question for people discovering this project is: How do I easily add
+desktop support to my existing Flutter application?
+
+The answer is that at this point, you don't. The project is still in early
+stages, and a lot of things are still in flux; if you don't already have
+experience doing desktop development on the platform(s) you want to add,
+this project is probably not ready for you to use it yet. The focus is currently
+on improving core functionality, not on ease of use. Neither the API surface nor
+the project structure are stable, and no attempt will be made to provide
+supported migration paths as things change.
+
+However, if you want to try out an existing Flutter application running on the
+desktop even with those caveats, and don't have experience with desktop
+development, here are two approaches that might work for you.
+
+With either approach, be sure to follow the [main README](README.md) and
+[library README](library/README.md) instructions on setting up prerequisites
+and adjusting your Flutter application.
+
+## Replace the 'example' Flutter Code
+
+Since `example/` is already configured to run on all the platforms this project
+supports, you can swap in your project's Dart code, `pubspec.yaml`, resources,
+etc., then follow the [normal directions](example/README.md) for building the
+example application on your platform.
+
+This will be the easiest approach to keep working as the project changes, but
+requires that you essentially wrap your whole application in a
+flutter-desktop-embedding checkout.
+
+## Copy the '\*\_fde' Directories
+
+Starting from the example projects means you don't have to create projects from
+scratch, and since they are self-contained they can be added to an existing
+project without needing to move it. However, because the projects build
+the flutter-desktop-embedding libraries from source, they contain relative paths
+to the flutter-desktop-embedding projects and tools they depend on. You will
+need to update those paths in order for the projects to work. On Linux, the
+variables you will need to change are documented in the Makefile. On macOS and
+Windows, you will need some familiarity with Xcode and Visual Studio
+respectively to make the changes.
+
+With this approach, you should expect breakage when you update the
+flutter-desktop-embedding reposity; when that happens you will need to look at
+what has changed in the exmaple projects and update your copies accordingly, or
+start over with fresh copies and adjust the paths again.

README.md

@@ -11,6 +11,9 @@ optional plugins to access other native platform functionality.
 
 ## How to Use This Code
 
+_If you have an existing Flutter app and just want to get it running, see
+the [quick start](Quick-Start.md) page before continuing._
+
 ### Setting Up
 
 The tooling and build infrastructure for this project requires that you have