Document post-3.1.0 API changes and BC scope

[Copilot]

Reviewed the 3.1.0..HEAD changeset to confirm there are no unintended breaking API removals beyond the expected PHP 8.0 floor and PHP 7 compatibility cleanup. Updated the docs so the README and changelog reflect the actual surface-area additions and fixes shipped since 3.1.0.

• BC scope

  • Clarifies that the intended breaking change in this range is the move to PHP 8.0+.
  • Notes that the request/response API changes since 3.1.0 are additive rather than removals.

• README refresh

  • Adds the newly introduced request helpers to the feature list, including async dispatch and curl-style alias methods.
  • Expands examples to show the newer fluent API for async requests and transfer metadata access.

• Changelog refresh

  • Rewrites the Unreleased section to match the actual code delta from 3.1.0 to HEAD.
  • Captures the new request helpers, response transfer-info helpers, direct-stream response fix, and the retry/NTLM fixes.

Example of the newly documented API shape:

$promise = \Httpful\Request::get('https://api.example.com/items')
    ->authenticateWithBearerToken('secret-token')
    ->sendAsync();

$response = $promise->wait();

echo $response->getEffectiveUrl() . "\n";
echo $response->getTransferHttpVersion() . "\n";

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This change is 

[coveralls]

coverage: 95.82%. remained the same — copilot/check-breaking-changes into master