Documentation on how to build the documentation

The Cookbook is a CMake target of the OTB project: it can be built by setting the BUILD_COOKBOOK cmake variable option to ON. The name of the target is CookbookHTML.

However the cookbook used to be a separate project (see this MR), with different build instruction. But the `README.md file has never been updated with the new instructions.

As far as i know, there is no up to date doc (wiki, cookbook ?) on how to build the Cookbook. I think updating the README.md file is enough.