Add compatibility note

README.md

@@ -23,6 +23,7 @@ Part of the Foreman installer: <https://github.com/theforeman/foreman-installer>
 | 2.x            | 1.5 - 1.10     |                                                     |
 | 1.x            | 1.4 and older  |                                                     |
 
+17.x switched to running `smart_proxy_dynflow` as part of `foreman-proxy` service by default. On EL* distributions and Foreman < 2.5, `foreman_proxy::plugin::dynflow::external_core` needs to be explicitly set to `true`.
 16.x added support for Smart Proxy Registration feature, available in Smart Proxy 2.3 and newer.
 12.x has dropped support for Puppet 3 which was officially unsupported for a while and Foreman Proxy 1.23 dropped altogether.