Add appendix for letters of collaboration

[bernhold]

Maybe use a csv file to organize things

[bernhold]

I had letters of collaboration on a recent proposal. I used the optional other-attachments.tex file that the template already understands. The content of the file was modeled on the bio and C&P appendices, except that I made no attempt to automate things with a csv file -- just had a \smartinclude line for each letter, entered and ordered manually.

I think there's still room to make this a little easier, but this was simple enough to implement that it isn't a priority for me. Pull requests welcome, however.