add minimal pgf version info in documentation and error in the package if package is out of date

README.md

 (chronograms). The package can be useful for drawing real-time timing
 diagrams in LaTeX, as often needs to be done for preparing articles,
 lectures and presentation in the Real-Time Scheduling research
-community. The package depends on keyval and TikZ/PGF (version 2.10 or greater).
+community. The package depends on keyval and TikZ/PGF (versions 2.10 or greater).
 
 A more complete documentation, with a set of examples, is available in
 file rtsched-doc.pdf.

rtsched-doc.tex

 package, which can be used to easily draw chronograms (GANTT charts).
 These diagrams are quite common in real-time scheduling research.
 
-The package depends on keyval and TikZ/PGF, both widely
+The package depends on keyval and TikZ/PGF (versions 2.10 or greater), both widely
 available on any \TeX distribution.
 
 The drawing capabilities are completely based on TikZ. Thus, you can compile