Add a README.

Originally, we pruned Android's fork to remove files which are not
needed by Android, and may be a distraction from the files which
matter.

We decided to include all of the files (to simplify merging), so
add a README.android to tell users to ignore the extraneous files.

BUG:http://skbug.com/2416
Change-Id: Ib34f355962f5e8b9a2eca6e9b9952baa277ca166

README.android

@@ -0,0 +1,13 @@
+The Skia library is a cross-platform 2D-based renderer. The
+key places to look are in the folders:
+
+  include/  : contains public header files
+  src/      : contains src code
+
+The top level makefile is at the root: Android.mk
+
+These files originate from an upstream repository
+(skia.googlesource.com) where the project is actively developed
+and maintained. As a result some files are not relevant to
+Android, but make the merging process smoother.
+