Add note to example about support for multiple CAs

examples/config.yaml

@@ -3,6 +3,7 @@
 
 # PKI defines the location of credentials for this node. Each of these can also be inlined by using the yaml ": |" syntax.
 pki:
+  # The CAs that are accepted by this node. Must contain one or more certificates created by 'nebula-cert ca'
   ca: /etc/nebula/ca.crt
   cert: /etc/nebula/host.crt
   key: /etc/nebula/host.key