add DS418play to compatability list

[seanauff]

Can confirm traffic is flowing after following the latest fixes in #10

$ uname -a
Linux Diskstation 4.4.59+ #24922 SMP PREEMPT Fri May 10 02:59:42 CST 2019 x86_64 GNU/Linux synology_apollolake_418play

$ sudo wg
interface: wg0
  public key: <redacted>
  private key: (hidden)
  listening port: 993

peer: <redacted>
  endpoint: <redacted>
  allowed ips: <redacted>
  latest handshake: 53 seconds ago
  transfer: 23.11 MiB received, 383.24 MiB sent

peer: <redacted>
  endpoint: <redacted>
  allowed ips: <redacted>
  latest handshake: 56 seconds ago
  transfer: 113.68 MiB received, 243.09 MiB sent

[runfalk]

Thank you!