Make it easy to install the tools to build the docs.

README

@@ -82,6 +82,11 @@ a makefile that uses nbconvert to generate ReST files from some of the
 notebooks, copies those to the sphinx source dir, then builds the HTML
 output using sphinx.
 
+Get the dependencies for (re)building the docs:
+
+    pip install Thun[build-docs]
+    make docs
+
 
 §.4 Basics of Joy
 

setup.py

@@ -47,5 +47,12 @@ setup(
 	install_requires=[
 		'dulwich',
 		'future',
-		]
+		],
+    extras_require={
+        "build-docs":  [
+			"sphinx",
+			"ipython",
+			"nbconvert",
+			],
+	    }
 	)