Clipboard: Add README.

Add a preliminary readme to help readers of the code quickly parse the
directory.

Change-Id: I9393282b5bd26a534ae141a4bb83d91da0504f18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2614119
Auto-Submit: Darwin Huang <huangdarwin@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840963}

ui/base/clipboard/README.md

+Platform-neutral clipboard abstractions, to access platform-specific clipboards
+(copy/paste) without platform-specific code.
+
+Interfaces include:
+* `Clipboard`: reading/pasting from the clipboard.
+* `ScopedClipboardWriter`: writing/copying to the clipboard.
+* `ClipboardObserver`: notifications of clipboard events.
+* `ClipboardFormatType`: specifying clipboard formats.
+
+While most platform-specific behavior should be abstracted away, some may
+still be exposed. For some notable platform-specific behavior exposed by these
+interfaces:
+* `ClipboardAndroid` has a more limited set of supported formats.
+* `ClipboardObserver` is only supported on some platforms, as other platforms
+  may require (inefficient) polling to implement.
+* `ClipboardX11` supports both the usual clipboard buffer, as well as the
+  selection (middle-click) paste buffer.
+* `DataTransferPolicyController` is only currently exercised in ChromeOS.