Update README.md: Create ConfigOverride.xcconfig by command line

README.md

@@ -24,9 +24,16 @@ git clone --branch=<<branch>> --recurse-submodules https://github.com/nightscout
 cd Trio
 ```
 
-modify Config.xconfig to replace ##TEAM_ID## with your Apple Developer ID.
+Create an ConfigOverride.xcconfig file that will automate signing of the build targets in Xcode:
 
 ```
+echo 'DEVELOPER_TEAM = xxxxxxxxxx' > ConfigOverride.xcconfig
+```
+
+Modify the Config.xconfig to replace `xxxxxxxxxx` with your Apple Developer ID. You can do this from within Xcode, or by using some other text editor.
+
+Then launch Xcode and build the Trio app:
+```
 xed .
 ```