Update readme for supporting opencl drivers on Ubuntu 18.04

[ayush1999]

ayush99@ayush99:~/beignet/build$ sudo apt-get install cmake pkg-config python ocl-icd-dev libegl1-mesa-
dev ocl-icd-opencl-dev libdrm-dev libxfixes-dev libxext-dev llvm-3.6-dev clang-3.6 libclang-3.6-dev
libtinfo-dev libedit-dev zlib1g-devD

Gives me a number on unable to locate package error:

E: Unable to locate package llvm-3.6-dev
E: Couldn't find any package by glob 'llvm-3.6-dev'
E: Couldn't find any package by regex 'llvm-3.6-dev'
E: Unable to locate package libclang-3.6-dev
E: Couldn't find any package by glob 'libclang-3.6-dev'
E: Couldn't find any package by regex 'libclang-3.6-dev'
E: Unable to locate package zlib1g-devD