Update README about gh-pages deployment and remove unused Slack workflow

Author: tberkel

.github/workflows/slack_notification.yaml

@@ -59,45 +59,16 @@ Finally to serve the website, you can run:
  python -m http.server --directory build/html
 ```
 
+### Viewing a forked repo GitHub Pages
 
-### Adding Content to GitHub Pages
-
-If you want to show the latest version of the docs on GitHub pages, build using then instructions above. Then from `pyscf-doc/source` run the following:
-
-```bash
-make gh_pages_setup
-```
+The PySCF website is currently built and deployed to GitHub Pages any time a push is made to the master branch of the repository.
+The website is served out of the `docs/` directory of the `gh-pages` branch.
+This is all controlled by a GitHub Action in `.github/workflows/docs_deploy.yaml`.
+If you forked this repository and you want to view the website on your own GitHub Pages, make sure to enable Actions on your repository,
+because they are disabled in forks by default.
 
 ## How to Contribute
 
 1.  Add a rst file \"your\_method.rst\" in the [source/user](source/user/) directory in which one describes the basic theory and usage of the method. Reference \"user/your\_method.rst\" in the \"toctree\" section in [source/user.rst](source/user.rst).
 2.  Add a rst file \"your\_module.rst\" in the [source/modules](source/modules/) directory in which one lists the examples and the member classes and functions of the module (the API doc is then generated by autodoc). (In the \"\_\_init\_\_.py\" file of each module, one should include a simple usage section. See [pyscf.dft.\_\_init\_\_.py](https://github.com/pyscf/pyscf/blob/master/pyscf/dft/__init__.py) as an example.) Reference \"your\_module.rst\" in the \"toctree\" section in [source/modules.rst](source/modules.rst).
 3.  Optionally, one could also add a rst file \"your\_method\_develop.rst\" in the  [source/develop](source/develop/) directory where one provides more detailed descriptions of the implementation and advanced guidelines for using and further development of the module. Reference \"your\_method\_develop.rst\" in [source/develop.rst](source/develop.rst). 
-
-
-<!-- ## Adding Blog Posts
-
-Create a new `.md` file in `pyscf-doc/source/posts` and add the following header (modified for your post):
-
-```
----
-blogpost: true
-date: February 1, 2021
-author: James Smith
-location: World
-category: Tutorial
-tags: HF, DFT, MCSCF
-language: English
----
-```
-
-If you want to write a post in `.rst` that's fine too! Just use the following in your header:
-
-```
-:blogpost: true
-:date: Oct 10, 2020
-:author: Nabil Freij
-:location: World
-:category: Manual
-:language: English
-``` -->