Explain how the script locates the HTML source

Author: leobalter

README.md

@@ -32,7 +32,13 @@ Run the `build.sh` script from inside your `html-build` working directory, like
 ./build.sh
 ```
 
-The first time this runs, it will ask for your input on where to clone the HTML source from, or where on your system to find it if you've already done that. If you're working to submit a pull request to [whatwg/html](https://github.com/whatwg/html), be sure to give it the URL of your fork.
+The first time this runs, it will look up for the HTML source from a `../html` folder, if it exists. Otherwise, it may ask for your input on where to clone the HTML source from, or where on your system to find it if you've already done that. If you're working to submit a pull request to [whatwg/html](https://github.com/whatwg/html), be sure to give it the URL of your fork.
+
+You may also set the environment variable `$HTML_SOURCE` to use a custom location for the HTML source. For example:
+
+```bash
+HTML_SOURCE=~/hacks/dhtml ./build.sh
+```
 
 ## Building using a Docker container