Updated the README documentation compilation.

doc/README

-Type the following in order to generate cta.pdf:
+The main file for CTA documentation is cta.tex. It can be compiled to PDF using
+TeXlive version 2015 or newer. Please not that an additional package should be downloaded
+before compiling (tikz-uml, see below).
 
-    pdflatex cta.tex
+In order to simplify things, the compiled pdf is also committed (and should be after
+updates of the tex files).
+
+tikz-uml: This package is not part of some latex distributions (like tex live), and 
+we are using a recent copy of it. It can be downloaded from the author's web page:
+% http://perso.ensta-paristech.fr/~kielbasi/tikzuml/