Merge pull request InsightSoftwareConsortium#4295 from thewtex/more-d…

…oc-build-instructions

DOC: Improve documentation contribution documents

Documentation/docs/README.md

@@ -1,18 +1,51 @@
-# Sphinx Documentation
+## How to contribute documentation improvements
 
-This is the source of the [ITK docs](docs.itk.org) website.
+Thank you for improving our documentation! Every addition made is read
+thousands of times and has a tremendous impact.
 
-## Build environment
+These are the sources for the [ITK docs](docs.itk.org)
+[sphinx](https://www.sphinx-doc.org/) website.
+
+The contribution process generally follows our [contributing
+guidelines](../../CONTRIBUTING.md).
+
+To preview documentation changes, three options are available.
+
+### Option 0: Edit on GitHub, use the pull request preview.
+
+If files are edited with GitHub's web user interface, the pull request will
+build a preview of changes with a pull request check called `docs/readthedocs.org:itk`.
+Click on the *Details* link to view the documentation build preview.
+
+### Option 1: Build and serve locally
 
 To compile the document locally create a python virtual environment and install the required packages.
 
 For example in Linux / macOS:
 
-```
+```sh
+cd ITK/Documentation/docs
 python -m venv env
 source env/bin/activate
 pip install -r requirements.txt
 ```
 
 Use `make html` in this directory to build the documentation.
 Open `_build/html/index.html` in your browser to inspect the result.
+
+### Option 2: Autobuild and serve locally
+
+To automatically rebuild the website with any input markdown changes and serve
+the result, use [sphinx-autobuild]
+
+```sh
+pip install sphinx-autobuild
+```
+
+```
+cd ITK/Documentation/docs
+sphinx-autobuild -a . _build/html
+```
+
+This will start a server at [http://127.0.0.1:8000](http://127.0.0.1:8000)
+and rebuild whenever the documentation changes.