Added doc on how to write http cache drivers

Author: vidarl

docs/drivers.md

@@ -0,0 +1,66 @@
+# Driver support
+
+You may add integration with other http caches using the extension points provided by this bundle.
+
+The following extension points are available
+ - PurgeClient
+ - TagHandler
+ - FOS TagHandler
+
+If you write a new PurgeClient driver, you **must** also create a corresponding TagHandler and vise
+versa. Creating a FOS TagHandler is optional.
+
+
+## PurgeClient
+
+The PurgeClient is responsible for sending purge requests to the http cache when content is about to be invalidated.
+The PurgeClient must implement EzSystems\PlatformHttpCacheBundle\PurgeClient\PurgeClientInterface and can be registered
+with the following code in service.yml:
+
+```
+services:
+    ezplatform.http_cache_myhttpcachebundle.purge_client.myhttpcache:
+        class: EzSystems\PlatformMyHttpCacheBundle\PurgeClient\MyHttpCachePurgeClient
+        arguments: ["@ezplatform.http_cache.cache_manager"]
+        tags:
+            - {name: ezplatform.http_cache.purge_client, purge_type: myhttpcache}
+```
+
+Any service which implements the PurgeClientInterface must be tagged with `ezplatform.http_cache.purge_client` in order
+to be registered as such.
+`purge_type` specifies what the value of the purge_type setting in `app/config/ezplatform.yml` should be in order to
+enable this driver.
+
+
+## TagHandler
+
+The TagHandler is responsible for tagging responses with headers which the http cache recognizes.
+The TagHandler must implement EzSystems\PlatformHttpCacheBundle\Handler\TagHandlerInterface and can be registered with
+the following code in service.yml:
+
+```
+    ezplatform.http_cache_myhttpcachebundle.tag_handler.myhttpcache:
+        class: EzSystems\PlatformMyHttpCacheBundle\Handler\MyHttpCacheTagHandler
+        tags:
+            - {name: ezplatform.http_cache.tag_handler, purge_type: myhttpcache}
+
+```
+
+Any service which implements the TagHandlerInterface must be tagged with `ezplatform.http_cache.tag_handler` in order
+to be registered as such.
+
+## FOS TagHandler
+
+The FOS Http cache bundle also have a TagHandler which is not used by eZ Platform except for one thing, the
+`fos:httpcache:invalidate:tag` command. With this command you may explicit invalidate cache by tag.
+
+Normally, you would not need to implement your own FOS TagHandler as the ezplatform-http-cache bundle ships with a
+default one which uses the PurgeClient to invalidate the given tags.
+If you anyway need to write your own FOS TagHandler, you may register it with the following code in service.yml:
+
+```
+    ezplatform.http_cache_myhttpcachebundle.fos_tag_handler.myhttpcache:
+        class: EzSystems\PlatformMyHttpCacheBundle\Handler\MyHttpCacheFosTagHandler
+        tags:
+            - {name: ezplatform.http_cache.fos_tag_handler, purge_type: myhttpcache}
+```