Update installation.rst (#102)

Just a suggestion, to improve the installation experience ;)

Author: kristbaum

doc/installation.rst

@@ -38,7 +38,7 @@ a manual compile. Download the file and run::
 
      $ p7zip -d snap7-full-1.0.0.7z  # requires the p7 program
      $ cd build/<platform>           # where platform is unix or windows
-     $ make -f <arch>.mk             # where arch is your architecture, for example x86_64_linux
+     $ make -f <arch>.mk install     # where arch is your architecture, for example x86_64_linux
 
 For more information about or help with compilation please check out the
 documentation on the `snap7 website <http://snap7.sourceforge.net/>`_.