[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.