README.md: Fix min LLVM version

Author: kinke

README.md

@@ -75,7 +75,7 @@ libraries is available on the project wiki for
 [Windows](http://wiki.dlang.org/Building_and_hacking_LDC_on_Windows_using_MSVC).
 
 If you have a working C++/D build environment, CMake, and a recent LLVM
-version (≥ 11) available, there should be no big surprises. Do not
+version (≥ 15) available, there should be no big surprises. Do not
 forget to make sure the Phobos submodule is up to date:
 
     $ cd ldc