Update contributing guide

Author: Meghan Jones

doc/contributing.md

@@ -322,12 +322,9 @@ General guidelines for making a good gallery plot:
 The tutorials (the User Guide in the docs) are also built by sphinx-gallery from the
 `.py` files in the `examples/tutorials` folder of the repository. To add a new tutorial:
 
-* Include a `.py` file in the `examples/tutorials` folder on the base of the repository.
+* Create a `.py` file in the `examples/tutorials/advanced` folder.
 * Write the tutorial in "notebook" style with code mixed with paragraphs explaining what
   is being done. See the other tutorials for the format.
-* Include the tutorial in the table of contents of the documentation (side bar). Do this
-  by adding a line to the User Guide `toc` directive in `doc/index.rst`. Notice that the
-  file included is the `.rst` generated by sphinx-gallery.
 * Choose the most representative figure as the thumbnail figure by adding a comment line
   `# sphinx_gallery_thumbnail_number = <fig_number>` to any place (usually at the top)
   in the tutorial. The *fig_number* starts from 1.