add to README that this is a header only lib

[psteinbrecher]

It is not clear from readme that his is a header only lib.

Add this to README for steps on how to use it directly from github:
git clone https://github.com/oneapi-src/oneDPL.git
export DPLROOT=${PWD}/oneDPL
export CPATH="${DPLROOT}/include:${CPATH}"