[Documentation] Clarify the variables that affect installation

Describe the bug The Makefile is very unclear about the destination directory.

To Reproduce Extract the tarball and run "make" and then "make install" seems to try to create files in the system directories.

Expected behavior There should be a DESTDIR target or a configure script or perhaps even some autotools/automake/autoconf input that allows a configure script to exist.

Desktop (please complete the following information): Red Hat Enterprise Linux 6 with gcc 12.1.0 and no CFLAGS other than -std=iso9899:2011 -fno-builtin -g -O0 -m64

Build system is whatever you provide in the tarball. The Makefile.

Software that is compiled and then tested with some testsuite should be optionally installed into a non-system directory.

I see a DESTDIR here :

https://github.com/facebook/zstd/blob/9a5e73c74ef2d621992154306ab1ab6ba44ac8fa/lib/Makefile#L259

However that is not in the main Makefile.