website: make it clearer how to retrieve DO key info

Author: mitchellh

website/source/docs/providers/do/r/droplet.html.markdown

@@ -34,7 +34,9 @@ The following arguments are supported:
 * `ipv6` - (Optional) Boolean controlling if IPv6 is enabled.
 * `private_networking` - (Optional) Boolean controlling if private networks are enabled.
 * `ssh_keys` - (Optional) A list of SSH IDs or fingerprints to enable in
-   the format `[12345, 123456]`
+   the format `[12345, 123456]`. To retrieve this info, use a tool such
+   as `curl` with the [DigitalOcean API](https://developers.digitalocean.com/#keys),
+   to retrieve them.
 
 ## Attributes Reference