add frontend readme

Signed-off-by: Jörn Friedrich Dreyer <jfd@butonic.de>

services/frontend/README.md

@@ -0,0 +1,27 @@
+# Frontend Service
+
+The frontend service translates various owncloud related HTTP APIs to CS3 requests. 
+
+## Endpoints Overview
+
+Currently, the frontend service handles request for three functionalities, which are `appprovider`, `archiver`, `datagateway` and `ocs`.
+
+### appprovider
+
+The appprovider endpoint, by default `/app`, forwards HTTP requests to the CS3 [App Registry API](https://cs3org.github.io/cs3apis/#cs3.app.registry.v1beta1.RegistryAPI)
+
+### archiver
+
+The archiver endpoint, by default `/archiver`, implements zip and tar download for collections of files. It will internally use the CS3 API to initiate downloads and then stream the individual files as part of a compressed file.
+
+### datagateway
+
+The datagateway endpoint, by default `/data`, forwards file up and download requests to the correct CS3 data provider. OCIS starts a dataprovider as part of the storage-* services. The routing happens based on the JWT that was created by a storage provider in response to an `InitiateFileDownload` or `InitiateFileUpload` request.
+
+### ocs
+
+The ocs endpoint, by default `/ocs`, implements the ownCloud 10 Open Collaboration Services API into CS3 API requests. It can handle users, groups, capabilities and also implements the files sharing functionality on top of CS3. The `/ocs/v[12].php/cloud/user/signing-key` is currently handled by the dedicated [ocs](https://github.com/owncloud/ocis/tree/master/services/ocs) service.
+
+## Scalability
+
+While the frontend service does not persist any data it does cache stat and information. Therefore, multiple instances of this service can be spawned in a bigger deployment like when using container orchestration with Kubernetes, when configuring `FRONTEND_OCS_RESOURCE_INFO_CACHE_TYPE=redis` and the related config options.