More clarity in docus about what "make install" will do, and where. #4757

I built zstd for the first time last night, on Gnu/Linux, and I want to point something out.
The docs, as many as there are, don't explain that on a supported platform, the binary products (lib and zstd) will go under /usr/local. Yes, that's something many people will assume, and that's what it does. But I asked myself what could be done if I wanted to install zstd somewhere else. Or what if I wanted to stage the install (put the installables into a temporary directory for eventual installation somewhere else).

Let me note that I admire the Makefile. It's very unusual these days to find that the build infrastructure is just a Makefile, rather than one which uses those other approaches (you do provide). Years ago I wrote Makefiles like this. Attempts to use every feature of gmake ;-). But I grepped through the Makefile and failed to locate anything like PREFIX or DESTDIR. Thus leaving me with the conclusion that using gmake alone, there was no way to stage.

How to address this
Introduce the DESTDIR and PREFIX variables into the Makefile logic.