Add a README.md

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

README.md

+This is a minimal Maven project implementing an ImageJ 1.x plugin
+
+It is intended as an ideal starting point to develop new ImageJ 1.x plugins
+in an IDE of your choice. You can even collaborate with developers using a
+different IDE than you.
+
+In [Eclipse](http://eclipse.org), for example, it is as simple as
+_File>Import...>Existing Maven Project_
+
+In [Netbeans](http://netbeans.org), it is even simpler: _File>Open_
+Project. The same works in [IntelliJ](http://jetbrains.net).
+
+If [jEdit](http://jedit.org) is your preferred IDE, you will need the [Maven
+Plugin](http://plugins.jedit.org/plugins/?MavenPlugin).
+
+Die-hard command-line developers can use Maven directly by calling _mvn_
+in the project root.
+
+However you build the project, in the end you will have the .jar file in the
+_target/_ subdirectory. Simply copy it into your ImageJ's _plugins/_
+directory, run _Help>Refresh Menus_ and the plugin will be available in
+_Process>Process Pixels_ (this can be changed by editing
+_src/main/resources/plugins.config_).