Add note on finding publication dates by looking at source code

[tlongers]

Update Research Handbook with this good example of looking in the source code to find the date:

URL: http://www.odogwublog.com/2014/04/obiano-launches-operation-kpochapu-with.html

• No publication date stated in the article itself
• Infer April 2014 from URL /2014/04/
• Look at the source and find this in the headers:

<abbr class='published timeago' itemprop='datePublished' title='2014-04-28T20:32:00+01:00'>Monday, April 28, 2014</abbr></a>