Add dynamic import maps example

Closes #55.

README.md

@@ -604,6 +604,54 @@ See [the proto-spec](./spec.md) for more details on how this all works.
 
 _What do we do in workers? Probably `new Worker(someURL, { type: "module", importMap: ... })`? Or should you set it from inside the worker? Should dedicated workers use their controlling document's map, either by default or always? Discuss in [#2](https://github.com/domenic/package-name-maps/issues/2)._
 
+### Dynamic import map example
+
+The above rules mean that you _can_ dynamically generate import maps, as long as you do so before performing any imports. For example:
+
+```html
+<script>
+const im = document.createElement('script');
+im.type = 'importmap';
+im.textContent = JSON.stringify({
+  imports: {
+    'my-library': Math.random() ? '/my-awesome-library.mjs' : '/my-rad-library.mjs';
+  }
+});
+document.currentScript.after(im);
+</script>
+
+<script type="module">
+import 'my-library';
+</script>
+
+A more realistic example might use this capability to override a previous import map based on feature detection:
+
+```html
+<script type="importmap">
+{
+  "imports": {
+    "lodash": "/lodash.mjs",
+    "moment": "/moment.mjs"
+  }
+}
+</script>
+
+<script>
+if (!someFeatureDetection()) {
+  const im = document.createElement('script');
+  im.type = 'importmap';
+  im.textContent = '{ "imports": { "lodash": "/lodash-legacy-browsers.js" } }';
+  document.currentScript.after(im);
+}
+</script>
+
+<script type="module">
+import _ from "lodash"; // will get the right thing
+</script>
+```
+
+Note that (like other `<script>` elements) modifying the contents of a `<script type="importmap">` after it's already inserted in the document will not work; this is why we wrote the above example by inserting a second `<script type="importmap">` to overwrite the first one.
+
 ### Scope
 
 Import maps are an application-level thing, somewhat like service workers. (More formally, they would be per-module map, and thus per-realm.) They are not meant to be composed, but instead produced by a human or tool with a holistic view of your web application. For example, it would not make sense for a library to include a import map; libraries can simply reference modules by specifier, and let the application decide what URLs those specifiers map to.