Add workaround (#86)

Add some tips for Ubuntu with llvm-6 and clang-6.

Add some notes from #74 .

README.md

@@ -67,7 +67,9 @@ Compiling the generator on Linux
 
 You need:
  - The clang libraries version 3.4 or later
-
+
+You may want to `sudo apt install llvm-7 clang-7 libclang-7-dev` on Ubuntu if you ran into error like that clang says it cannot find "ClangConfig.cmake". More details in [issues#74](https://github.com/woboq/woboq_codebrowser/issues/74) .
+
 Example:
 ```bash
 cmake . -DCMAKE_BUILD_TYPE=Release