figure legend and image update

[raynamharris]

Added a legend for figure 1. In one sentence, I state the rule and some little extra bit of information that ties the image in the figure to the explanatory information in the body of the manuscript.

Figure modifications.

1. Modified the image for rule 7 to incorporate a reviewer's suggestion (Figure 1 #42)
2. Removed the PLOS logo reference to our paper.
3. Made more space around rules 1-4.
4. Tweaked the icons for rules 3 and 6

[ianmilligan1]

Looks great to me too!