Added documentation for amazon_s3 adapter

I've added what I understand based on getting this working today. Not sure about all the options. 

I also added a note about bucket names with dots in them - this had me stuck for a while until I found mention of it in passing in an AWS ticket https://forums.aws.amazon.com/thread.jspa?threadID=122890#jive-message-445833

If it's not a generally repeatable problem feel free to delete that line, but it certainly caused me problems!

README.markdown

@@ -398,6 +398,64 @@ knp_gaufrette:
                 ttl: 0
 ```
 
+## Amazon S3 (amazon_s3)
+
+Adapter to connect to Amazon S3 instances.
+
+This adapter requires the use of amazonwebservices/aws-sdk-for-php which can be installed by adding the following line to your composer.json:
+
+```
+    "require": {
+        ...
+        "amazonwebservices/aws-sdk-for-php": "1.6.2"
+    },
+```
+
+Note that Gaufrette is not currently compatible with the v2 Amazon SDK (called "aws/aws-sdk-php").
+
+### Parameters
+
+ * `amazon_s3_id`: the id of the AmazonS3 service used for the underlying connection
+ * `bucket_name`: the name of the bucket to use
+ * `options`: additional (optional) settings
+   * `directory`: the directory to use, within the specified bucket
+   * `region`
+   * `create`
+
+### Defining services
+
+To use the Amazon S3 adapter you need to provide a valid `AmazonS3` instance (as defined in the Amazon SDK). This can 
+easily be set up as using Symfony's service configuration:
+
+``` yaml
+# app/config/config.yml
+services:
+    amazonS3:
+        class: AmazonS3
+        arguments:
+            options:
+                key:      '%aws_key%'
+                secret:   '%aws_secret_key%'
+```   
+
+### Example
+
+Once the service is set up use its key as the amazon_s3_id in the gaufrette configuration:
+
+``` yaml
+# app/config/config.yml
+knp_gaufrette:
+    adapters:
+        foo:
+            amazon_s3:
+                amazon_s3_id:   amazonS3
+                bucket_name:    foo_bucket
+                options:
+                    directory:  foo_directory
+```
+
+Note that the SDK seems to have some issues with bucket names with dots in them, e.g. "com.mycompany.bucket" seems to have issues but "com-mycompany-bucket" works.
+
 ## Open Cloud (opencloud)
 
 Adapter for OpenCloud (Rackspace)