Document new templates default includeDate option

[jviotti]

As described in jsdoc/jsdoc#910, I included a new templates.default.includeDate option that allows the user to disable the current date from the default template.

The PR (jsdoc/jsdoc#916) has been merged, so it'd be nice if this new option can get documented.

• If templates.default.includeDate is omitted, the default behaviour is preserved and the date is shown.
• If templates.default.includeDate is true, the default behaviour is preserved and the date is shown.
• If templates.default.includeDate is false, the current date is not shown.

[hegemonic]

Fixed on master.