docs: add custom framework finalize_reporters! and vips install notes

- Document how to call finalize_reporters! for custom test frameworks
- Add brew/apt install commands for libvips in Requirements

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: pftg

README.md

@@ -194,9 +194,20 @@ Comparisons add ~50ms per image with VIPS. Without `ruby-vips`, ChunkyPNG is use
 `DEBUG=1 bundle exec rake test` keeps `.diff.png` files for inspection.
 </details>
 
+## Custom Test Frameworks
+
+Minitest, RSpec, and Cucumber are supported out of the box. For other frameworks, call `finalize_reporters!` after all tests complete:
+
+```ruby
+# In your framework's "after suite" hook:
+CapybaraScreenshotDiff.finalize_reporters!
+```
+
+This generates the HTML report and prints the summary. Without this call, the gem falls back to `at_exit` which may fire before your tests finish.
+
 ## Installation
 
-**Requirements:** Ruby 3.2+. Rails 7.1+ for Rails integration; non-Rails projects supported via `CapybaraScreenshotDiff.serve()`. For the `:vips` driver: [libvips 8.9+](https://libvips.github.io/libvips/install.html).
+**Requirements:** Ruby 3.2+. Rails 7.1+ for Rails integration; non-Rails projects supported via `CapybaraScreenshotDiff.serve()`. For the `:vips` driver: [libvips 8.9+](https://libvips.github.io/libvips/install.html). On macOS: `brew install vips`. On Ubuntu: `apt-get install libvips-dev`.
 
 ## Docs