api [nfc]: Document minimum server version

The Zulip release lifecycle:
  https://zulip.readthedocs.io/en/latest/overview/release-lifecycle.html
prescribes 18 months of support after the next release is out.

Zulip Server 4.0 was released 19 months ago, so we don't support
any older release than that.

Author: gnprice

README.md

@@ -18,6 +18,10 @@ samples, guidance on mobile development, and a full API reference.
 
 ### Editing API types
 
+We support Zulip Server 4.0 and later.  For API features added in
+newer versions, use `TODO(server-N)` comments (like those you see
+in the existing code.)
+
 When editing the files in `lib/api/model/`, use the following command
 to keep the generated files up to date:
 ```