[canvaskit] Add MacOS specific notes for setup

Change-Id: Ie42f6c7c980f952bb18b4444716cbe4681507e9f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/294601
Reviewed-by: Leon Scroggins <scroggo@google.com>
diff --git a/modules/canvaskit/README.md b/modules/canvaskit/README.md
index a392d08..48a1970 100644
--- a/modules/canvaskit/README.md
+++ b/modules/canvaskit/README.md
@@ -6,6 +6,13 @@
 
 [1]: https://emscripten.org/docs/getting_started/downloads.html
 
+## MacOS specific notes
+Make sure you have Python3 installed, otherwise the downloading emscripten toolchain
+can fail with errors about SSL certificates. <https://github.com/emscripten-core/emsdk/pull/273>
+
+See also <https://github.com/emscripten-core/emscripten/issues/9036#issuecomment-532092743>
+for a solution to Python3 using the wrong certificates.
+
 # Compile and Run Local Example
 
 ```