regionset: include README in package

The README file contains most of the documentation for how to use this
package, and is explicitly referenced in the manual. Add it to the
compiled package so users can access it more easily, and make sure the
path to the file is correct, too.

pkgs/by-name/re/regionset/package.nix

···
     sha256 = "1fgps85dmjvj41a5bkira43vs2aiivzhqwzdvvpw5dpvdrjqcp0d";
   };
 
+  prePatch = ''
+    substituteInPlace regionset.8 \
+        --replace-fail /usr/share/doc/ "$out"/share/doc/
+  '';
+
   installPhase = ''
     install -Dm755 {.,$out/bin}/regionset
     install -Dm644 {.,$out/share/man/man8}/regionset.8
+    install -Dm644 {.,$out/share/doc/regionset}/README
   '';
 
   meta = with lib; {