infra: pepper some GNU on top of Makefile

Readme.org

@@ -32,6 +32,14 @@ If you want to make a self-contained binary, you'll need `chicken` with `libchic
   CSC_FLAGS=-static make
 #+end_src
 
+** Installation
+#+begin_src sh
+  # default installation (/usr/local/bin/)
+  make install
+  # you can customize the prefix (will install to ~/.local/bin)
+  make prefix=~/.local install
+#+end_src
+
 * Usage
 ** Summary
 #+begin_src