docs: networking: netdevsim rules

There are aspects of netdevsim which are commonly
misunderstood and pointed out in review. Cong
suggest we document them.

Suggested-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/netdev-FAQ.rst

@@ -228,6 +228,23 @@ before posting to the mailing list. The patchwork build bot instance
 gets overloaded very easily and netdev@vger really doesn't need more
 traffic if we can help it.
 
+netdevsim is great, can I extend it for my out-of-tree tests?
+-------------------------------------------------------------
+
+No, `netdevsim` is a test vehicle solely for upstream tests.
+(Please add your tests under tools/testing/selftests/.)
+
+We also give no guarantees that `netdevsim` won't change in the future
+in a way which would break what would normally be considered uAPI.
+
+Is netdevsim considered a "user" of an API?
+-------------------------------------------
+
+Linux kernel has a long standing rule that no API should be added unless
+it has a real, in-tree user. Mock-ups and tests based on `netdevsim` are
+strongly encouraged when adding new APIs, but `netdevsim` in itself
+is **not** considered a use case/user.
+
 Any other tips to help ensure my net/net-next patch gets OK'd?
 --------------------------------------------------------------
 Attention to detail.  Re-read your own work as if you were the