Updated building from scratch

INSTALL.txt

 INSTALLATION
 
 The python orekit wrapper requires:
-- a python 2.X installation
-- The JCC tool http://lucene.apache.org/pylucene/jcc/
+- a python installation (version 2.3-2.7)
+- The JCC tool version 2.19 or later http://lucene.apache.org/pylucene/jcc/
 - a java JDK
 
+The easiest way to install the orekit wrapper is to use pre-build packages.
+
+To build the wrapper from scratch:
+
+Place the java wrapper source file in the orekit source tree. Compile an
+orekit jar file including the wrapper.
+
+Place this jar file together with apache-commons-math jar file in the same
+directory as the provided build file, and execute the build. Note that
+the end command of the build may need to be change according to your needs,
+"install","bdist_winist" etc.
 
 Hints:
 One common failure is that the JCC package cannot reach the java virtual machine.