Update README.fuchsia to specify upstream branch.

Change-Id: I80b5826655a0d222fda8a7fd6ca896ced1b2214f
diff --git a/README.fuchsia b/README.fuchsia
index bf93f45..5d70ae2 100644
--- a/README.fuchsia
+++ b/README.fuchsia
@@ -11,6 +11,7 @@
 Contains support for applying an ICC color profile to an image.  The code is
 originally based on tinycms, re-written by Mozilla for better security and
 performance.  This copy is a source-drop from Mozilla on March 13, 2012.
+This branch is based off of upstream/v4.
 Local Modifications:
 Some files only have license headers in the master branch.