adding clang/llvm readme

Signed-off-by: Alex McCaskey <mccaskeyaj@ornl.gov>

tools/compiler/README.md

+
+## Building XACC with single-source compiler
+You will need to install the latest clang and llvm libraries and headers (Version 9). On Ubuntu, this can be done with the following commands
+```bash
+$ wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add -
+$ echo "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial main" > /etc/apt/sources.list.d/llvm.list
+$ apt-get update
+$ apt-get install -y libclang-9-dev llvm-9-dev
+$ ln -s /usr/bin/llvm-config-9 /usr/bin/llvm-config
+```