mac build: re-add note about needing full Xcode...

Turns out the Xcode CLI tools are still not enough, and we need full Xcode :(
This is only for notarization; as `altool` (which we need there) is only part
of full Xcode. So we need to download 8 gigs just for that single script...

related c1dbcab9bb

contrib/osx/README.md

@@ -31,7 +31,25 @@ We currently build the release binaries on macOS 10.14.6, and these seem to run
 
 Before starting, make sure that the Xcode command line tools are installed (e.g. you have `git`).
 
-#### Build Electrum
+
+#### 1. Get Xcode
+
+Notarizing the application requires full Xcode
+(not just command line tools as that is missing `altool`).
+
+Get it from [here](https://developer.apple.com/download/more/).
+Unfortunately, you need an "Apple ID" account.
+
+(note: the last Xcode that runs on macOS 10.14.6 is Xcode 11.3.1)
+
+After downloading, uncompress it.
+
+Make sure it is the "selected" xcode (e.g.):
+
+    sudo xcode-select -s $HOME/Downloads/Xcode.app/Contents/Developer/
+
+
+#### 2. Build Electrum
 
     cd electrum
     ./contrib/osx/make_osx