📝 add Xcode 12 to CI list

README.md

@@ -1226,6 +1226,7 @@ The following compilers are currently used in continuous integration at [Travis]
 | Apple Clang 10.0.1 (clang-1001.0.46.4); Xcode 10.2.1            | macOS 10.14.4      | Travis         |
 | Apple Clang 11.0.0 (clang-1100.0.33.12); Xcode 11.2.1           | macOS 10.14.6      | Travis         |
 | Apple Clang 11.0.3 (clang-1103.0.32.59); Xcode 11.4.1           | macOS 10.15.4      | GitHub Actions |
+| Apple Clang 12.0.0 (clang-1200.0.22.7); Xcode 11.4.1            | macOS 10.15.5      | Travis         |
 | Clang 3.5.0 (3.5.0-4ubuntu2~trusty2)                            | Ubuntu 14.04.5 LTS | Travis         |
 | Clang 3.6.2 (3.6.2-svn240577-1~exp1)                            | Ubuntu 14.04.5 LTS | Travis         |
 | Clang 3.7.1 (3.7.1-svn253571-1~exp1)                            | Ubuntu 14.04.5 LTS | Travis         |