expand-response-params: add description

Since the tool is exposed more prominently now, we should clear up what
it is and note that it is to be considered unstable, i.e. we may change
it if the necessity arises. (In practice it is probably going to be
fairly stable though, as compiler interfaces tend to be quite stable.)

Should we add a version?

pkgs/build-support/expand-response-params/default.nix

···
-{ stdenv }:
+{ stdenv, lib }:
 
 # A "response file" is a sequence of arguments that is passed via a
 # file, rather than via argv[].
···
     mkdir -p $prefix/bin
     mv expand-response-params $prefix/bin/
   '';
+
+  meta = {
+    description = "Internal tool used by the nixpkgs wrapper scripts for processing response files";
+    longDescription = ''
+      expand-response-params is a tool that allows for obtaining a full list of all
+      arguments passed in a given compiler command line including those passed via
+      so-called response files. The nixpkgs wrapper scripts for bintools and C
+      compilers use it for processing compiler flags. As it is developed in
+      conjunction with the nixpkgs wrapper scripts, it should be considered as
+      unstable and subject to change.
+    '';
+    license = lib.licenses.mit;
+    platforms = lib.platforms.all;
+  };
 }