doc: add experimental apis

Documentation/dev-guide/experimental_apis.md

@@ -0,0 +1,8 @@
+# Experimental APIs and features
+
+For the most part, the etcd project is stable, but we are still moving fast! We believe in the release fast philosophy. We want to get early feedback on features still in development and stabilizing. Thus, there are, and will be more, experimental features and APIs. We plan to improve these features based on the early feedback from the community, or abandon them if there is little interest, in the next few releases. If you are running a production system, please do not rely on any experimental features or APIs.
+
+## The current experimental API/features are:
+
+- v3 auth API: expect to be stale in 3.1 release
+- etcd gateway: expect to be stable in 3.1 release

Documentation/docs.md

@@ -14,6 +14,7 @@ The easiest way to get started using etcd as a distributed key-value store is to
  - [Interacting with etcd][interacting]
  - [API references][api_ref]
  - [gRPC gateway][api_grpc_gateway]
+ - [Experimental features and APIs][experimental]
 
 ## Operating etcd clusters
 
@@ -67,4 +68,4 @@ To learn more about the concepts and internals behind etcd, read the following p
 [understand_apis]: learning/api.md
 [versioning]: op-guide/versioning.md
 [supported_platform]: op-guide/supported-platform.md
-
+[experimental]: dev-guide/experimental_apis.md