Add meta-ffmpeg4 to layer depends and add hint where to find it

Signed-off-by: Andreas Müller <schnitzeltony@gmail.com>

conf/layer.conf

@@ -10,5 +10,11 @@ BBFILE_PRIORITY_meta-retro = "10"
 
 LICENSE_PATH += "${LAYERDIR}/files/licenses"
 
-LAYERDEPENDS_meta-retro = "core openembedded-layer filesystems-layer"
+# ffmpeg4-layer: https://github.com/schnitzeltony/meta-ffmpeg4
+LAYERDEPENDS_meta-retro = " \
+    core \
+    openembedded-layer \
+    filesystems-layer \
+    ffmpeg4-layer \
+"
 LAYERSERIES_COMPAT_meta-retro = "honister kirkstone"