Mention RSVG dependency when creating the disk image on OSX

doc/build-osx.md

@@ -18,6 +18,10 @@ Dependencies
 
     brew install automake berkeley-db4 libtool boost --c++11 miniupnpc openssl pkg-config homebrew/versions/protobuf260 --c++11 qt5 libevent
 
+In case you want to build the disk image with `make deploy` (.dmg / optional), you need RSVG
+
+    brew install librsvg
+
 NOTE: Building with Qt4 is still supported, however, could result in a broken UI. Building with Qt5 is recommended.
 
 Build Bitcoin Core