mac build: build qr scanner on separate machine

contrib/osx/README.md

@@ -8,12 +8,13 @@ This guide explains how to build Electrum binaries for macOS systems.
 
 This needs to be done on a system running macOS or OS X. We use El Capitan (10.11.6) as building it
 on High Sierra (or later)
-makes the binaries incompatible with older versions. 
+makes the binaries [incompatible with older versions](https://github.com/pyinstaller/pyinstaller/issues/1191).
 
+Before starting, make sure that the Xcode command line tools are installed (e.g. you have `git`).
 
-#### 1.1 Get Xcode
+#### 1.1a Get Xcode
 
-Building the QR code reader (CalinsQRReader) requires full Xcode (not just command line tools).
+Building the QR scanner (CalinsQRReader) requires full Xcode (not just command line tools).
 
 The last Xcode version compatible with El Capitan is Xcode 8.2.1
 
@@ -27,6 +28,17 @@ Make sure it is the "selected" xcode (e.g.):
 
     sudo xcode-select -s $HOME/Downloads/Xcode.app/Contents/Developer/
 
+#### 1.1b Build QR scanner separately on newer Mac
+
+Alternatively, you can try building just the QR scanner on newer macOS.
+
+On newer Mac, run:
+
+    pushd contrib/osx/CalinsQRReader; xcodebuild; popd
+    cp -r contrib/osx/CalinsQRReader/build prebuilt_qr
+
+Move `prebuilt_qr` to El Capitan: `contrib/osx/CalinsQRReader/prebuilt_qr`.
+
 
 #### 1.2 Build Electrum