docs: add docs on firefox private mode and IDB

Fixes #9

[skip ci]

README.md

@@ -62,6 +62,7 @@ A lightweight emoji picker, distributed as a web component.
     + [Data source and JSON format](#data-source-and-json-format)
     + [Trimming the emojibase data](#trimming-the-emojibase-data)
     + [Offline-first](#offline-first)
+    + [Environments without IndexedDB](#environments-without-indexeddb)
   * [Design decisions](#design-decisions)
     + [IndexedDB](#indexeddb)
     + [Native emoji](#native-emoji)
@@ -785,6 +786,14 @@ try {
 
 If `emoji-picker-element` fails to fetch the JSON data the first time it loads, then it will display an error message.
 
+### Environments without IndexedDB
+
+`emoji-picker-element` has a hard requirement on [IndexedDB](https://developer.mozilla.org/en-US/docs/Glossary/IndexedDB), and will not work without it.
+
+For browsers that don't support IndexedDB, such as [Firefox in private browsing mode](https://bugzilla.mozilla.org/show_bug.cgi?id=1639542), you can polyfill it using [fake-indexeddb](https://github.com/dumbmatter/fakeIndexedDB). Here is [a working example](https://bl.ocks.org/nolanlawson/651e6fbe4356ff098f505e6cc5fb8cd8) and [more details](https://github.com/nolanlawson/emoji-picker-element/issues/9).
+
+For Node.js environments such as [Jest](https://jestjs.io/) or [JSDom](https://github.com/jsdom/jsdom), you can also use fake-indexeddb. A [working example](https://github.com/nolanlawson/emoji-picker-element/blob/d45b61b559b8cef6840b7036e3d1749a213c490e/config/jest.setup.js#L15-L18) can be found in the tests for this very project.
+
 ## Design decisions
 
 Some of the reasoning behind why `emoji-picker-element` is built the way it is.
@@ -799,9 +808,6 @@ Using IndexedDB has a few advantages:
 2. After the first load, there is no need to download, parse, and index the JSON file again, because it's already available in IndexedDB.
 3. If you want, you can even [load the IndexedDB data in a web worker](https://github.com/nolanlawson/emoji-picker-element/blob/ff86a42/test/adhoc/worker.js), keeping the main thread free from non-UI data processing.
 
-Note that because `emoji-picker-element` has a requirement on IndexedDB, it will not work
-in enviroments where IDB is unavailable, such as [Firefox private browsing](https://bugzilla.mozilla.org/show_bug.cgi?id=1639542). See [issue #9](https://github.com/nolanlawson/emoji-picker-element/issues/9) for more details.
-
 ### Native emoji
 
 To avoid downloading a large sprite sheet that renders a particular emoji set – which may look out-of-place on different platforms, or may have [IP issues](https://blog.emojipedia.org/apples-emoji-crackdown/) – `emoji-picker-element` only renders native emoji. This means it is limited to the emoji actually installed on the user's device.

docs/index.html

@@ -163,9 +163,11 @@ <h1>emoji-picker-element</h1>
     <div class="private-browsing-warning"
          style="padding: 20px; display: none; border: 2px dashed crimson;"
          role="alert">
-      Note that <code>emoji-picker-element</code> currently does not support Firefox private browsing mode,
-      or any other setting that disables IndexedDB. See
-      <a href="https://github.com/nolanlawson/emoji-picker-element/issues/9">issue #9</a>.
+      Note that <code>emoji-picker-element</code> does not support environments without IndexedDB. For polyfills
+      and workarounds, see
+      <a href="https://github.com/nolanlawson/emoji-picker-element/blob/master/README.md#environments-without-indexeddb">
+        the README
+      </a>.
     </div>
   </div>
 </div>