Add advice for complying with licenses of embedded work to "Making READMEs readable"

[brittag]

Also fixed an outdated URL on that page.

[wslack]

LGTM, nice work! Defer to someone else to merge, as I'm not primary on this guide.

[mbland]

Also LGTM, but @melodykramer should probably be the one to do the honors.

This also reminds me that, as the .about.yml schema matures, we should perhaps make references to it from this guide as well. (I was reminded because licenses is one of the fields.) Perhaps we could even make a tool that could parse the .about.yml and update the README based on the advice from the guide. (Hint: We can put <div id="licenses">...</div> in the README and inject .about.yml information that way.)

cc: @jeremiak for the .about.yml parsing idea.