Split README into a bunch of files in docs/

docs/configuring-playbook-s3.md

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+# Amazon S3 configuration (optional)
+
+By default, this playbook configures your server to store Matrix 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 Matrix Synapse's content repository (`media_store`) files on Amazon S3,
+you can let this playbook configure [Goofys](https://github.com/kahing/goofys) for you.
+
+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
+{
+	"Version": "2012-10-17",
+	"Statement": [
+		{
+			"Sid": "Stmt1400105486000",
+			"Effect": "Allow",
+			"Action": [
+				"s3:*"
+			],
+			"Resource": [
+				"arn:aws:s3:::your-bucket-name",
+				"arn:aws:s3:::your-bucket-name/*"
+			]
+		}
+	]
+}
+```
+
+You then need to enable S3 support in your configuration file (`inventory/matrix.<your-domain>/vars.yml`).
+It would be something like this:
+
+```yaml
+matrix_s3_media_store_enabled: true
+matrix_s3_media_store_bucket_name: "your-bucket-name"
+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"
+```