Adding serial letter example.

For http://tex.stackexchange.com/a/215958/828, entirely based on
http://de.wikibooks.org/wiki/LaTeX-W%C3%B6rterbuch:_Serienbrief
Updated credits
Adding example on how to combine several PDFs

My answer for http://tex.stackexchange.com/q/218215/828
Merge branch 'master' of https://github.com/habi/latex
Merge pull request #1 from habi/master

update