Add some README info about the docs repo

README.md

@@ -64,6 +64,20 @@ Once the `get` completes, you should find your new `hugo` (or `hugo.exe`) execut
 To update Hugo’s dependencies, use `go get` with the `-u` option.
 
     go get -u -v github.com/gohugoio/hugo
+
+## The Hugo Documentation
+
+The Hugo documentation now lives in its own repository, see https://github.com/gohugoio/hugoDocs. But we do keep a version of that documentation as a `git subtree` in this repository. To build the sub folder `/docs` as a Hugo site, you need to clone this repo with submodules:
+
+```bash
+git clone --recursive git@github.com:gohugoio/hugo.git
+```
+
+Or after you have cloned it you can do:
+
+```bash
+git submodule update --init
+```
 
 ## Contributing to Hugo