Update README.md, mention new --source argument

README.md

@@ -6,11 +6,12 @@ file, mirroring its remote images, stylesheets, and other resources.
 
 ## Usage
 ```
-usage: mirror-img [-h] [-d DIR] [-b BASE] HTML_FILE
-       mirror-img [-h] [-d DIR] [-b BASE]
+usage: mirror-img [-h] [-d DIR] [-b BASE] [-s URL] HTML_FILE
+       mirror-img [-h] [-d DIR] [-b BASE] [-s URL]
 Available options:
   -h, --help          print this help text.
-  -b, --base          path to mirror directory used in URLs
+  -b, --base ARG      path to mirror directory used in URLs
+  -s, --source ARG    URL used to resolve & mirror relative URLs
   -d, --downloads ARG directory for all mirrored files
 ```
 
@@ -21,12 +22,28 @@ In order to mirror a webpage, you can simply download it and pipe it into mirror
 $ curl https://www.gnu.org/philosophy/philosophy.html | mirror-img > philosophy.html
 ```
 
-And now `philosophy.html` will be a fully-local HTML file, with no external resources!
-All mirrored content will be found in the `mirror/` directory, and all links
-have been adjusted accordingly.
+And now `philosophy.html` is a fully-local HTML file with no external resources!
+
+… at least, it *would* be. Notice how some resources, like the CSS, don’t load.
+This is because they are defined as *relative* links (e.g., “../style.css”
+rather than “https://invalid.tld/style.css”). In order for these to be
+mirrored as well, mirror-img needs to somehow know the source URL.
+
+You can use the `--source` argument to provide the source URL, so
+relatively-linked resources can be mirrored, too:
+
+```
+$ SOURCE_URL="https://www.gnu.org/philosophy/philosophy.html"
+$ curl "$SOURCE_URL" | mirror-img --source "$SOURCE_URL" > philosophy.html
+```
+
+*Now* we’re done! All mirrored content will be found in the `mirror/`
+directory, and all links have been adjusted accordingly.
+
+---
 
 If you’d like to change the download directory, you can use the `--downloads`
-argument. To change the directory used in the output HTML’s URLs, you can
+argument. To change the directory used in the output-HTML’s URLs, you can
 use `--base`.
 
 For example, if you’d like to mirror files into `/tmp/mirrors/` but have URLs