Added note 100 s) on default binding of IP.diff

tools/hdltool_readme.txt

@@ -1029,8 +1029,20 @@ toolset scheme needs to be improved because:
 - the toolsets are tight to a board name 'unb1' (is that oke?) or should we use more general toolset names, or do we need a symbolic toolset names at all? 
 - there is also a 'site' level in the bash scripts (is that still needed?)
 
+s) Declare IP libraries to ensure default binding in simulation.
+
+Currently the IP library is declared in the technology VHDL file e.g. like 'LIBRARY ip_arria10_ddr4_4g_1600_altera_emif_150;' in tech_ddr_arria10.vhd.
+This IP library clause is ignored by synthesis. The IP library must be mapped for simulation, because otherwise Modelsim gives
+an error when it compiles the VHDL. Therefore the IP library can then not be excluded with hdl_lib_excludes key.
+Alternatively the LIBRARY clause could be omitted if the IP library is added to the -L libraries search list of each simulation configuration the
+Modelsim project file. This can be achieved adding the IP library to the modelsim_search_libraries key in the hdltool_unb2.cfg. However the problem is
+then that if the IP library is not mapped to a directory then Modelsim will issue an error when it tries to search it.
+The unb2_test uses almost all IP libraries and then the revision uses hdl_lib_excludes to exclude the IP libraries that are not needed. It is not 
+necessary to exclude all IP libraries that are not used, but at least the ones that will cause errors if their qip/sdc file is sourced.
+--> For now keep the hdl_lib_excludes but only use it for synthesis, for simulation the hdl_lib_excludes is ignored. 
    
 
+
 101) More ideas
 
 a) zip scripts