Email-reports example - add an example/explanation for private plots with share_key

[priyatharsan]

It would be better to have an example/explanation regarding how to create magic urls from private links with share_key and use for sending email-reports
For example:
Something like this:

from IPython.display import display, HTML

template = (''
    '<a href="{graph_url}" target="_blank">' # Open the interactive graph when you click on the image
        '<img src="{graph_url}">'        # Use the ".png" magic url so that the latest, most-up-to-date image is included
    '</a>'
    '{caption}'                              # Optional caption to include below the graph
    '<br>'                                   # Line break
    '<a href="{graph_url}" style="color: rgb(190,190,190); text-decoration: none; font-weight: 200;" target="_blank">'
        'Click to comment and see the interactive graph'  # Direct readers to Plotly for commenting, interactive graph
    '</a>'
    '<br>'
    '<hr>'                                   # horizontal line
'')

email_body = ''
graph = 'https://plot.ly/~chelsea_lyn/17461.png?share_key=3kCBg9awEny15vobuAP5Up'
_ = template
_ = _.format(graph_url=graph, caption='')
email_body += _

display(HTML(email_body))