Updating contributor guidelines.

CONTRIBUTING.md

@@ -1,7 +1,30 @@
-Want to contribute? Great! First, read this page (including the small print at
-the end).
+# Contributing
 
-### Before you contribute
+Want to contribute to the Flutter sample ecosystem? Great! First, read this
+page (including the small print at the end).
+
+## Is this the right place for your contribution?
+
+This repo is used by members of the Flutter team and a few partners as a place
+to store example apps and demos. It's not meant to be the one and only source of
+truth for Flutter samples or the only place people go to learn about the best
+ways to build with Flutter. What that means in practice is that if you've
+written a great example app, it doesn't need to be maintained here in order to
+get noticed, be of help to new Flutter devs, and have an impact on the
+community.
+
+You can maintain your sample app in your own repo (or with another source
+control provider) and still be as important a part of the Flutter-verse as
+anything you see here. You can let us know on the
+[FlutterDev Google Group](https://groups.google.com/forum/#!forum/flutter-dev)
+when you've published something and Tweet about it with the
+[#flutterio](https://twitter.com/search?q=%23flutterio) hashtag.
+
+## So what should be contributed here, then?
+
+Fixes and necessary improvements to the existing samples, mostly.
+
+## Before you contribute
 
 Before we can use your code, you must sign the
 [Google Individual Contributor License Agreement](https://cla.developers.google.com/about/google-individual)
@@ -18,19 +41,19 @@ us first through the issue tracker with your idea so that we can help out and
 possibly guide you. Coordinating up front makes it much easier to avoid
 frustration later on.
 
-### Code reviews
+## Code reviews
 
 All submissions, including submissions by project members, require review.
 
-### File headers
+## File headers
 
 All files in the project must start with the following header.
 
-    // Copyright 2016 The Chromium Authors. All rights reserved.
+    // Copyright 2018 The Chromium Authors. All rights reserved.
     // Use of this source code is governed by a BSD-style license that can be
     // found in the LICENSE file.
 
-### The small print
+## The small print
 
 Contributions made by corporations are covered by a different agreement than the
 one above, the