add paragraph for subversion and about NetCDF descriptor

README

@@ -17,4 +17,15 @@ Bugs:
 -----
  See file BUGS in main tsgqc directory
 
+Subversion:
+-----------
+The code is maintained under revision source control software subversion under:
+https://svn.mpl.ird.fr/us191/tsg-qc/trunk/
+
+You can get last update local copy with svn update.
+
+If you try to modify code, never change or modify by hand the NetCDF descriptor
+file @netcdf_native/tsgqc_netcdf.csv
+
+