Since you are reading this, you are on the syntax-macros branch.
This branch contains the LLVM sources to support the Clang syntax
macros extension.
If you are looking for the standard LLVM readme, it is located in the file README.txt in this folder.
Source code for Clang with the syntax macros extension can be obtained from https://github.com/normanrink/clang-syntax-macros.