update readme to mention submodule
update readme to mention submodule

OpenAstronomy/openastronomy.github.io,Cadair/openastronomy.github.io,OpenAstronomy/openastronomy.github.io,dpshelio/openastronomy.github.io,OpenAstronomy/openastronomy.github.io,Cadair/openastronomy.github.io,Cadair/openastronomy.github.io,dpshelio/openastronomy.github.io,dpshelio/openastronomy.github.io