Provide advice in the README about aliasing `xxd`

[philiptaron]

In nixpkgs, we're contemplating switching over from the original Vim-sourced xxd to this xxd.

• https://github.com/NixOS/nixpkgs/pull/335277

The PR brought some discussion about whether the packaging for tinyxxd ought to directly include an alias for xxd or whether a secondary output should be used.

The argument for including it is that Arch packaging does so.

The argument against it is that nothing in this repository (especially the Makefile) does anything of the sort.

Could you please clarify in the README your expectation for packagers?

[xyproto]

Hi!

• My goal is for this utility to replace xxd in connection with Linux/BSD/Homebrew/etc packaging.
• My (perhaps unrealistic) dream is that ViM unbundles xxd from ViM and supports this lean standalone fork instead.

I'll update the README too.