[Plugin] Add documentation for the VCR plugin (#262)

* Add documentation for the VCR plugin

plugins/index.rst

@@ -26,3 +26,4 @@ request or you can even start a completely new request. This gives you full cont
    request-uri-manipulations
    retry
    stopwatch
+   vcr

plugins/vcr.rst

@@ -0,0 +1,104 @@
+VCR Plugin - Record and Replay Responses
+========================================
+
+The VCR plugins allow you to record & replay HTTP responses. It's very useful for test purpose (using production-like predictable fixtures and avoid making actual HTTP request).
+You can also use it during your development cycle, when the endpoint you're contacting is not ready yet.
+
+Unlike the :doc:`php-http/mock-client </clients/mock-client>`, where you have to manually define responses, the responses are **automatically** generated from the previously recorded ones.
+
+Install
+-------
+
+.. code-block:: bash
+
+    $ composer require --dev php-http/vcr-plugin
+
+Usage
+-----
+
+To record or replay a response, you will need two components, a **naming strategy** and a **recorder**.
+
+The naming strategy
+*******************
+
+The naming strategy turn a request into a deterministic and unique identifier.
+The identifier must be safe to use with a filesystem.
+The plugin provide a default naming strategy, the ``PathNamingStrategy``. You can define two options:
+
+* **hash_headers**: the list of header(s) that make the request unique (Ex: 'Authorization'). The name & content of the header will be hashed to generate a unique signature. By default no header is used.
+* **hash_body_methods**: indicate for which request methods the body makes requests distinct. (Default: PUT, POST, PATCH)
+
+This naming strategy will turn a GET request to https://example.org/my-path to the ``example.org_GET_my-path`` name, and optionally add hashes if the request
+contain a header defined in the options, or if the method is not idempotent.
+
+To create your own strategy, you need to create a class implementing ``Http\Client\Plugin\Vcr\NamingStrategy\NamingStrategyInterface``.
+
+The recorder
+************
+
+The recorder records and replays responses. The plugin provides two recorders:
+
+* ``FilesystemRecorder``: Saves the response on your filesystem using Symfony's `filesystem component`_ and `Guzzle PSR7`_ library.
+* ``InMemoryRecorder``: Saves the response in memory. **Response will be lost at the end of the running process**
+
+To create your own recorder, you need to create a class implementing the following interfaces:
+
+* ``Http\Client\Plugin\Vcr\Recorder\RecorderInterface`` used by the RecordPlugin.
+* ``Http\Client\Plugin\Vcr\Recorder\PlayerInterface`` used by the ReplayPlugin.
+
+The plugins
+***********
+
+There are two plugins, one to record responses, the other to replay them.
+
+* ``Http\Client\Plugin\Vcr\ReplayPlugin``, use a ``PlayerInterface`` to replay previously recorded responses.
+* ``Http\Client\Plugin\Vcr\RecordPlugin``, use a ``RecorderInterface`` instance to record the responses,
+
+Both plugins add a response header to indicate either under which name the response has been stored (RecordPlugin, ``X-VCR-RECORD`` header), or which response name has been used to replay the request (ReplayPlugin, ``X-VCR-REPLAYED`` header).
+
+If you plan on using both plugins at the same time (Replay or Record), the ``ReplayPlugin`` **must always** come first.
+Please also note that by default, the ``ReplayPlugin`` throws an exception when it cannot replay a request. If you want the plugin to continue the request (possibly to the actual server), set the third constructor argument to ``false`` (See example below).
+
+Example
+*******
+
+.. code-block:: php
+
+    <?php
+
+    use Http\Client\Common\PluginClient;
+    use Http\Client\Plugin\Vcr\NamingStrategy\PathNamingStrategy;
+    use Http\Client\Plugin\Vcr\Recorder\FilesystemRecorder;
+    use Http\Client\Plugin\Vcr\RecordPlugin;
+    use Http\Client\Plugin\Vcr\ReplayPlugin;
+    use Http\Discovery\HttpClientDiscovery;
+
+    $namingStrategy = new PathNamingStrategy([
+        'hash_headers' => ['X-Custom-Header'], // None by default
+        'hash_body_methods' => ['POST'], // Default: PUT, POST, PATCH
+    ]);
+    $recorder = new FilesystemRecorder('some/dir/in/vcs'); // You can use InMemoryRecorder as well
+
+    // To record responses:
+    $record = new RecordPlugin($namingStrategy, $recorder);
+
+    // To replay responses:
+    // Third argument prevent the plugin from throwing an exception when a request cannot be replayed
+    $replay = new ReplayPlugin($namingStrategy, $recorder, false);
+
+    $pluginClient = new PluginClient(
+        HttpClientDiscovery::find(),
+        [$replay, $record] // Replay should always go first
+    );
+
+    /** @var \Psr\Http\Message\RequestInterface $request */
+    $request = new MyRequest('GET', 'https://httplug.io');
+
+    // Will be recorded in "some/dir/in/vcs"
+    $client->sendRequest($request);
+
+    // Will be replayed from "some/dir/in/vcs"
+    $client->sendRequest($request);
+
+.. _filesystem component: https://symfony.com/doc/current/components/filesystem.html
+.. _Guzzle PSR7: https://github.com/guzzle/psr7