Merge pull request #537 from robotology/doc-xcode-12

Add note on Xcode 12 compatibility

README.md

@@ -142,6 +142,9 @@ xcodebuild [-configuration Release|Debug] [-jobs <n>] [-list | -target <target_n
 ```
 `-list` gives the list of available targets.
 
+**Note: as of late 2020, the Xcode 12 generator is not supported, for more info see https://github.com/robotology/ycm/issues/368. 
+  All previous versions instead should work fine. If you have Xcode 12 installed in your macOS system, please use the GNU Makefiles generator.**
+
 ### Configure your environment
 The superbuild provides an automatically generated `setup.sh` sh script that will set all the necessary enviromental variables to use the software installed in the robotology-superbuild. To do so automatically for any new terminal that you open, append the following line to the `.bash_profile` file:
 ```