Update third_party/fastfeat/README.libaom

Add a note about the modification introduced in commit 1c9752b9

Change-Id: I23e32441d888dc1cd1286de5f0e51086dea99f9a
diff --git a/third_party/fastfeat/README.libaom b/third_party/fastfeat/README.libaom
index ce7ce70..d658f99 100644
--- a/third_party/fastfeat/README.libaom
+++ b/third_party/fastfeat/README.libaom
@@ -39,3 +39,4 @@
 Convert tabs to spaces
 Prefix global functions with "aom_"
 Add error checking
+Add output argument to hold the scores of the detected features