Add figure_ids parameter to README.

README.md

@@ -56,6 +56,8 @@ The argument "plot_engine" defines which plot engine was used to create the figu
 tagplot returns a PlotIDTransfer object that contains the tagged figures and the corresponding IDs as strings.
 
 Optional parameters can be used to customize the tag process.
+- *figure_ids*: list of str, optional
+        IDs that will be printed on the plot. If empty, IDs will be generated for each plot. If this option is used, an ID for each plot has to be specified. Default: [].
 - *prefix* : str, optional
         Will be added as prefix to the ID.
 - *id_method* : str, optional

plotid/tagplot.py

@@ -41,7 +41,8 @@ def tagplot(
     ----------------
     figure_ids: list of str, optional
         IDs that will be printed on the plot. If empty, IDs will be generated for each
-        plot. Default: [].
+        plot. If this option is used, an ID for each plot has to be specified.
+        Default: [].
     location : str, optional
         Location for ID to be displayed on the plot. Default is "east".
     rotation: float or int, optional