Top-level introductory paragraph for multi-framework documentation in OSS.

PiperOrigin-RevId: 347653240

tensorflow_federated/experimental/BUILD

+package(default_visibility = ["//visibility:private"])
+
+licenses(["notice"])

tensorflow_federated/experimental/README.md

+This directory contains experimental code that is being incubated. Do not depend
+on this code from other parts of TFF, as it can change at any time. All code
+included here should retain private visibility while in the incubation stage.

tensorflow_federated/experimental/g3doc/multiframework.md

+# Multi-Framework Support in TensorFlow Federated
+
+TensorFlow Federated (TFF) has been designed to support a broad range of
+federated computations, expressed through a combination of TFF's federated
+operators that model distributed communication, and local processing logic.
+
+Currently local processing logic can be expressed using TensorFlow APIs (via
+`@tff.tf_computation`) at the frontend, and is executed via the TensorFlow
+runtime at the backend. However, we aim to support multiple other
+(non-TensorFlow) frontend and backend frameworks for local computations,
+including non-ML frameworks (e.g., for logic expressed in SQL or general-purpose
+programming languages).
+
+In this section, we'll include information on:
+
+*   Mechanisms that TFF provides to support alternative frameworks, and how you
+    can add support for your preferred type of frontend or backend to TFF.
+
+*   Experimental implementations of support for non-TensorFlow frameworks, with
+    examples.
+
+*   Tentative future roadmap for graduating these capabilities beyond the
+    experimental status.