Improve documentation regarding mailchimp settings

[benlk]

We're test driving the Link Roundup plugin and are trying to connect it to Mailchimp via the documentation, however there's no field to add the API key in the options area.

Never mind. Mailchimp settings appeared at the bottom of my dashboard menu. Once it was populated the default settings panel now appears in the Link Roundup submenu, and the individual post settings are at the bottom of the post. Documentation needs updating.

So:

• [ ] In the Link Roundups settings, put a link to the Mailchimp SDK settings.
• [ ] Update plugin docs to make sure that "configure mailchimp if you want to" is very obvious post-install
• [ ] Update Mailchimp-regarding docs to make the location of the Mailchimp configuration more obvious

[benlk]

Partially addressed in https://github.com/INN/link-roundups/pull/139