mac build: note in README: pyinstaller picks up extraneous brew pkgs

ThomasV had libffi installed via brew, and this side-effected the built electrum dmg.
`('libffi.8.dylib', '/usr/local/opt/libffi/lib/libffi.8.dylib', 'BINARY')`

this was the cause of 019d213325

contrib/osx/README.md

@@ -57,6 +57,9 @@ Before starting, you should install [`brew`](https://brew.sh/).
     $ xcrun --show-sdk-path
     /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk
     ```
+- Installing extraneous brew packages can result in build differences.
+  For example, pyinstaller seems to pick up and bundle brew-installed `libffi`.
+  So having a dedicated "electrum binary builder macOS VM" is recommended.
 - Make sure that you are building from a fresh clone of electrum
   (or run e.g. `git clean -ffxd` to rm all local changes).