Add docs about using other S3-compatible object stores

docs/configuring-playbook-s3.md

@@ -3,9 +3,11 @@
 By default, this playbook configures your server to store Synapse's content repository (`media_store`) files on the local filesystem.
 If that's alright, you can skip this.
 
-If you'd like to store Synapse's content repository (`media_store`) files on Amazon S3,
+If you'd like to store Synapse's content repository (`media_store`) files on Amazon S3 (or other S3-compatible service),
 you can let this playbook configure [Goofys](https://github.com/kahing/goofys) for you.
 
+## Amazon S3
+
 You'll need an Amazon S3 bucket and some IAM user credentials (access key + secret key) with full write access to the bucket. Example security policy:
 
 ```json
@@ -37,3 +39,13 @@ matrix_s3_media_store_aws_access_key: "access-key-goes-here"
 matrix_s3_media_store_aws_secret_key: "secret-key-goes-here"
 matrix_s3_media_store_region: "eu-central-1"
 ```
+
+
+## Using other S3-compatible object stores
+
+You can use any S3-compatible object store by **additionally** configuring these variables:
+
+```yaml
+matrix_s3_media_store_custom_endpoint_enabled: true
+matrix_s3_media_store_custom_endpoint: "your-custom-endpoint"
+```