update description of plugins

plugins/README.md

@@ -2,8 +2,16 @@
 
 A collection of additional plugins for demonstrating some features of the MorphoLibJ library.
 
+### Installation
+
 To install the plugins:
 * download the required jar file
 * copy it into the "plugins" folder of your ImageJ/Fiji installation
 * restart ImageJ or run the "refresh menus" action
 * the selected plugin(s) will appear in the menu "Plugins->MorphoLibJ Plus"
+
+### Contents
+
+* **Directional_Filters.jar**: contains the plugins "Directional Filter" and "Orientation Stack", both in "MorphoLibJ Plus" sub-menu.
+* **Grayscale_Granulometry.jar**: contains the plugins "Grayscale Granulometry", in "Granulometry" sub-menu. More information on [the GitHub page of the project](https://github.com/ijtools/ijGranulometry)
+* **Morphological_Reconstruction_Demo.jar**: contains the plugin "Morphological Reconstruction Demo", in "MorphoLibJ Plus" sub-menu.