README.md file in TSP-pstl

The README file in day4/examples/TSP-pstl explains
how to build the TSP example (travelling salesman
problem) using parallel STL and C++ modules. Since,
among other things, it needs and the Clang compiler
at of at least version 16.0, compiled with support
for parallel algorithms, and that is not the default
behaviour for simple "cmake; make; make install"
builds of Clang, the README file explains in detail
how to build a suitable version of Clang with support
for parallel STL.