Update yasm download instruction

BUG=aomedia:2511

Change-Id: I201d622bc374e5adb347e910a7508cd262cce7ec
diff --git a/README.md b/README.md
index 6b58d35..e8b06e1 100644
--- a/README.md
+++ b/README.md
@@ -47,7 +47,9 @@
  2. [Git](https://git-scm.com/).
  3. [Perl](https://www.perl.org/).
  4. For x86 targets, [yasm](http://yasm.tortall.net/), which is preferred, or a
-    recent version of [nasm](http://www.nasm.us/).
+    recent version of [nasm](http://www.nasm.us/). If you download yasm with
+    the intention to work with Visual Studio, please download win32.exe or
+    win64.exe and rename it into yasm.exe. DO NOT download or use vsyasm.exe.
  5. Building the documentation requires [doxygen](http://doxygen.org).
  6. Building the unit tests requires [Python](https://www.python.org/).
  7. Emscripten builds require the portable