Clarify components/README about when to create a component.

BUG=624590

Review-Url: https://codereview.chromium.org/2166093002
Cr-Commit-Position: refs/heads/master@{#407075}

components/README

-This directory is for features that are intended for reuse across multiple
-embedders (e.g., Android WebView and Chrome).
+This directory is for features that are intended for reuse. Example use cases:
+-code that is shared by Chrome on iOS and other Chrome platforms (since the iOS
+  port doesn't use src/chrome)
+-code that is shared between multiple embedders of content (e.g., Android
+  WebView and Chrome)
+-code that is shared between Blink and the browser process (since code in the
+  browser doesn't use Blink, while Blink doesn't include content or chrome to
+  avoid circular dependencies)
+
+In general, if some code is used by a directory "foo" and things above "foo" in
+the dependency tree, the code should probably live in "foo".
 
 By default, components can depend only on the lower layers of the Chromium
-codebase(e.g. base/, net/, etc.). Individual components may additionally allow
+codebase (e.g. base/, net/, etc.). Individual components may additionally allow
 dependencies on the content API and IPC; however, if such a component is used
 by Chrome for iOS (which does not use the content API or IPC), the component
 will have to be in the form of a layered component