[gimp-macos-build] Add infromation about notarization step

[Alex Samorukov <asamorukov src gnome org>]

commit 9a541348026cc85147ccc808d03e0aca280b5ade
Author: Alex Samorukov <samm os2 kiev ua>
Date:   Thu Nov 7 09:13:45 2019 +0100

    Add infromation about notarization step

 README.md | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/README.md b/README.md
index fd0093a..fb8ef41 100644
--- a/README.md
+++ b/README.md
@@ -35,6 +35,7 @@ I was able to get working builds in the VirtualBox VM, it works stable enough fo
   - compiles all `.py` files to `.pyc` to avoid writes to the Application folder
   - Signing all binaries
   - Creating a DMG package using [create-dmg](https://github.com/andreyvit/create-dmg) tool and signing it 
and signing it
+- Notarizing package using Apple `altool` utility
 - Uploading a DMG to the CircleCI build artifacts
 
 ## Other related links