MAINT: fix gitmodules and add README

Author: jakevdp

.gitmodules

@@ -0,0 +1,6 @@
+[submodule "website/plugins/ipynb"]
+path = website/plugins/ipynb
+url = git://github.com/danielfrg/pelican-ipynb.git
+[submodule "website/plugins/pelican-plugins"]
+path = website/plugins/pelican-plugins
+url = git://github.com/getpelican/pelican-plugins.git