macos build: brew now requires xcode cli tools (not full xcode)

Which means we need *both* xcode cli tools and full xcode.
Document this weirdness, including the exact paths they should be at.

excerpt from terminal when running make_osx:
```
Warning: You are using macOS 10.14.
We (and Apple) do not provide support for this old version.
You will encounter build failures with some formulae.
Please create pull requests instead of asking for help on Homebrew's GitHub,
Twitter or any other official channels. You are responsible for resolving
any issues you experience while you are running this
old version.

==> Downloading https://ftp.gnu.org/gnu/coreutils/coreutils-9.0.tar.xz
Already downloaded: /Users/vagrant/Library/Caches/Homebrew/downloads/5744bb33344b6180adca9d909a87e830d55982a1b3229c61e9dc0e35cfacbf25--coreutils-9.0.tar.xz
Error: Xcode alone is not sufficient on Mojave.
Install the Command Line Tools for Xcode 11.3.1 from:
  https://developer.apple.com/download/all/
```

contrib/osx/README.md

@@ -40,20 +40,20 @@ Before starting, you should install `brew`.
   to the user's home dir: `/Users/vagrant/electrum`.
 - Builders need to use the same version of Xcode; and note that
   full Xcode and Xcode commandline tools differ!
-  You should build with Xcode 11.3.1 (full Xcode).
-  The path for Xcode should be exactly as follows:
+  - Xcode CLI tools are sufficient for everything, except it is missing `altool`,
+    which is needed for the release-manager to notarise the `.dmg`.
+  - so full Xcode is needed, to have `altool`.
+  - however, brew now consider macOS 10.14 too old, and for some reason it
+    requires Xcode CLI tools. (`Error: Xcode alone is not sufficient on Mojave.`)
+  
+  So, we need *both* full Xcode and Xcode CLI tools. Both with version 11.3.1.
+  The two Xcodes should be located exactly as follows:
     ```
     $ xcode-select -p
     /Users/vagrant/Downloads/Xcode.app/Contents/Developer
     $ xcrun --show-sdk-path
-    /Users/vagrant/Downloads/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk
+    /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk
     ```
-  Note: make sure neither command above refers to the Xcode command line tools!
-  If so, rename the cli tools, e.g.
-    ```
-    $ mv /Library/Developer/CommandLineTools /Library/Developer/CommandLineTools2
-    ```
-  As a sanity check, make sure `$ gcc --version` consistently refers to the full Xcode.
 - Make sure that you are building from a fresh clone of electrum
   (or run e.g. `git clean -ffxd` to rm all local changes).
 
@@ -68,6 +68,14 @@ Unfortunately, you need an "Apple ID" account.
 
 (note: the last Xcode that runs on macOS 10.14.6 is Xcode 11.3.1)
 
+The recommended versions are:
+```
+$ shasum -a 256 "$HOME/Downloads/Xcode_11.3.1.xip"
+9a92379b90734a9068832f858d594d3c3a30a7ddc3bdb6da49c738aed9ad34b5  /Users/vagrant/Downloads/Xcode_11.3.1.xip
+$ shasum -a 256 "$HOME/Downloads/Command_Line_Tools_for_Xcode_11.3.1.dmg"
+1c4b477285641cca5313f456b712bf726aca8db77f38793420e1d451588673f9  /Users/vagrant/Downloads/Command_Line_Tools_for_Xcode_11.3.1.dmg
+```
+
 After downloading, uncompress it.
 
 Make sure it is the "selected" xcode (e.g.):