Add a "Description:" line to README.fuchsia

Necessary to conform to the format at
https://fuchsia.googlesource.com/docs/+/master/development/source_code/readme_fuchsia_format.md

Change-Id: Ib4f5c5b09bf747cacd1e832dc40d859aba0be4fe
diff --git a/README.fuchsia b/README.fuchsia
index a094709..0b2e771 100644
--- a/README.fuchsia
+++ b/README.fuchsia
@@ -4,6 +4,7 @@
 License File: CREDITS.fuchsia
 Upstream Git: git://source.ffmpeg.org/ffmpeg.git
 Last Upstream Merge: 6a97ba521558ce131fe839eed2d51ff745280e1d, Feb 11 2018
+Description:
 
 This file documents the layout of the Fuchsia copy of FFmpeg, some common
 tasks, and how to create the build files and related configurations.