substantially revise README with instructions to update the doc

Author: bqminh

README.md

@@ -1,2 +1,83 @@
-This is the source code for the main website of IQ-TREE.
-If you want to contribute to the documentation of IQ-TREE, there are several markdown file in the `doc` folder. Simply clone this repo, and make a pull request if you want to contribute. Thank you!
+Introduction
+============
+
+This repository stores the "source code" for the main IQ-TREE website at
+<https://iqtree.github.io>. Changes to this repo will automatically show up
+in the main website via GitHub pages support.
+
+We welcome you to contribute to the website! For example, you can improve the
+documentation at <https://iqtree.github.io/doc/> by updating _markdown_ files
+within the [doc](doc/) folder of the repo.
+[Markdown](https://en.wikipedia.org/wiki/Markdown) is a  human-readable text
+file format, which is very simple and convenient to edit. If you are teaching
+IQ-TREE at a workshop, you can update the markdown files in the
+[workshop](workshop/) folder, which will then show up at
+<https://iqtree.github.io/workshop/>. Below are the guidelines how to
+contribute.
+
+First, you should clone the repo to your local machine using git:
+
+```
+git clone https://github.com/iqtree/iqtree.github.io.git
+```
+
+This will download a local copy of the repo in the sub-folder `iqtree.github.io`
+of your current folder. This step needs to be done only ONCE! You can then
+follow the sections below for the two common tasks: [updating a
+documentation](#updating-iq-tree-documentation) or [updating a workshop](#updating-a-workshop). 
+
+
+Updating IQ-TREE documentation
+------------------------------
+
+The names of the files in [doc](doc/) folder are self-explanatory. For example,
+[doc/Tutorial.md](doc/Tutorial.md) corresponds to
+<https://iqtree.github.io/doc/Tutorial>. Each markdown file contains a header
+and the main text. Once you update the main text, make sure that you also update
+the header, especially the two fields: `author` to add your name (if it does
+appear in the author list yet) and `date` to the current date. For example,
+
+```
+---
+layout: userdoc
+title: "Beginner's Tutorial"
+author: Minh Bui, Your Name
+date:   YYYY-MM-DD
+...
+---
+```
+
+If you add a new section in the doc, make sure that you also add
+the `sections` part of the header, for example:
+
+```
+---
+....
+sections:
+- name: Your section title
+  url: your-section-title
+---
+```
+
+where the `url` field should be all in lower cases and empty spaces are replaced
+by a dash (`-`).
+
+Once you are satisfied with the changes you can then commit it:
+
+```
+git commit -am "Message about this commit"
+```
+
+Please make the message concise but informative about what you did. Then you
+can make a pull request to ask us to update the main repo. An IQ-TREE team
+member will review your pull request before deciding to merge it into the main
+site, optional modifications to your pull request. Occasionally, the
+IQ-TREE member may ask you to revise your pull request, before making decision.
+
+Updating a workshop
+-------------------
+
+The workshop materials are in the [workshop](workshop/) subfolder. There are already
+some past workshops that you can have a look.
+
+