The JCrypTool Doc project contains all available JCrypTool documentation like the Getting started with JCrypTool guide and other useful information for (crypto) plug-in development.
Have a look at our wiki for more information on how to get you started as a JCrypTool developer. Feel free to ask any question you might have in our gitter chat.
JCrypTool downloads are available as weekly builds (unstable, built automatically based on the develop branch) and as release builds (stable).
General information on the CrypTool and JCrypTool projects is available in the CrypTool portal.