Add duck mascot to README

Exclude assets/ from the installed gem, alongside the already-excluded
test/ and spec/ directories.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: thiagoa

README.md

@@ -2,6 +2,12 @@
 
 [![CI](https://github.com/thoughtbot/duck_typer/actions/workflows/ci.yml/badge.svg)](https://github.com/thoughtbot/duck_typer/actions/workflows/ci.yml)
 
+<div align="center">
+  <img alt="DuckTyper mascot" src="assets/duck_mugshot.png" width="300">
+</div>
+
+> If it quacks like a duck, it's a duck. Or is it a duck in disguise?
+
 DuckTyper enforces duck-typed interfaces in Ruby by comparing the
 public method signatures of classes, surfacing mismatches through
 your test suite.

assets/duck_mugshot.png

@@ -25,7 +25,7 @@ Gem::Specification.new do |spec|
   spec.files = IO.popen(%w[git ls-files -z], chdir: __dir__, err: IO::NULL) do |ls|
     ls.readlines("\x0", chomp: true).reject do |f|
       (f == gemspec) ||
-        f.start_with?(*%w[bin/ test/ spec/ features/ .git appveyor Gemfile])
+        f.start_with?(*%w[assets/ bin/ test/ spec/ features/ .git appveyor Gemfile])
     end
   end
   spec.bindir = "exe"