Add warnings to synapse-s3-storage-provider support feature

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## synapse-s3-storage-provider support
 
+**`synapse-s3-storage-provider` support is very new and still relatively untested. Using it may cause data loss.**
+
 You can now store your Synapse media repository files on Amazon S3 (or another S3-compatible object store) using [synapse-s3-storage-provider](https://github.com/matrix-org/synapse-s3-storage-provider) - a media provider for Synapse (Python module), which should work faster and more reliably than our previous [Goofys](docs/configuring-playbook-s3-goofys.md) implementation (Goofys will continue to work).
 
 This is not just for initial installations. Users with existing files (stored in the local filesystem) can also migrate their files to `synapse-s3-storage-provider`.

docs/configuring-playbook-synapse-s3-storage-provider.md

@@ -3,6 +3,8 @@
 If you'd like to store Synapse's content repository (`media_store`) files on Amazon S3 (or other S3-compatible service),
 you can use the [synapse-s3-storage-provider](https://github.com/matrix-org/synapse-s3-storage-provider) media provider module for Synapse.
 
+**`synapse-s3-storage-provider` support is very new and still relatively untested. Using it may cause data loss.**
+
 An alternative (which has worse performance) is to use [Goofys to mount the S3 store to the local filesystem](configuring-playbook-s3-goofys.md).