Added description for modelsim_search_libraries key.diff

tools/hdltool_readme.txt

@@ -277,6 +277,11 @@ d) hdllib.cfg key descriptions
     Global root path (e.g. $HDL_BUILD_DIR) or local library path (e.g. '') to the build directory for simulation. The path gets extended with the
     tool_name_synth from hdltool.cfg.
 
+- modelsim_search_libraries =
+    List of IP technology search libraries that will be put in the -L {} option of a Modelsim simulation configuration in the mpf. This avoids that all
+    IP technology needs to be compiled into the work library. The -L {} option is needed for simulations in libraries that use generated IP like
+    ip_stratixiv_phy_xaui which do not recognize the IP technology libraries mapping in [libraries] section in the mpf.
+
 - modelsim_copy_files =
     Copy listed all directories and files for simulation with Modelsim, used when tool_name_sim = modelsim in hdltool.cfg.