Add README clarifications

Version 5 is no longer supported; bumping up to version 8.

With newer versions of Java we need to use Mockito >=2.18.0

New optional fields are supported allowing the client to get and set the
preheader and amp_html fields on a TemplateVersionDetails object.

The amp_html field is the AMP-enabled version of an email; in cases
where AMP is not supported the email will fall back to either the HTML
or text version of the template.

The resend test was failing sometimes because the sendlog wouldn't exist
when the test tried to resend the email. Adding a 5 second wait is
hack-y but effective until we can remove our dependency on the live
production API.

Add preheader and amp_html support

Sometimes we may want to additional information to a target ESP through the
`headers` field. This change updates Headers from being a HashMap of <String,
String> to the more generic <String, Object>

Add complex header support

Bump version number

Adds another version of the `render` method that accepts a Map in which
any top-level `render` param can be specified.

- Send users to the docs page instead of the github markdown
- rename param to something more generic (since this isn't a send)

`render`: allow passing any param (#56)

Adds a new render() method that takes a hashmap instead of specific fields.

v3.1.0