README: add information about zscanner configuration

README

@@ -89,6 +89,9 @@ Install Knot DNS into system:
 $ sudo make install
 $ sudo ldconfig
 
+NOTE: If you want to reduce zscanner compile time add to configure --disable-fastparser,
+or --enable-fastparser=-T0 for the simplest variant (make check complains in this case).
+
 Running
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