Update README.md

docs/README.md

@@ -50,8 +50,10 @@ build the documentation with:
 mdbook build
 ```
 
-The rendered contents will be outputted to a new `book/` directory at the root of the repository. You can
-browse the book by opening `book/index.html` in a web browser.
+The rendered contents will be outputted to a new `book/` directory at the root of the repository. Please note that 
+index.html is not built by default, it is created by copying over the file `welcome_and_overview.html` to `index.html`
+during deployment. Thus, when running `mdbook serve` locally the book will initially show a 404 in place of the index
+due to the above. Do not be alarmed!
 
 You can also have mdbook host the docs on a local webserver with hot-reload functionality via: