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

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.