Add some documentation

frontend/README.md

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+# LibreSpeed frontend
+
+This is a re-implementation of the LibreSpeed user interface based on [the
+design by fromScratch](https://github.com/librespeed/speedtest/issues/585). The
+code that's doing the actual speed test is still the same.
+
+### Desktop
+
+![Desktop screenshot](./screenshots/desktop.png)
+
+### Mobile
+
+![Mobile screenshot](./screenshots/mobile.png)
+
+## How to use
+
+Copy everything in this directory into the LibreSpeed root, next to
+`speedtest.js` and `speedtest_worker.js`, overwriting the `index.html` file that
+may already be there. That's it!
+
+## Configuration
+
+In the [`server-list.json`](./server-list.json) file you can provide a list of
+testing servers. If you only have a single testing server, just provide a list
+with one item in it, being your server. The frontend will then skip doing an
+automatic server selection and will not allow the user to change servers.
+
+For more advanced applications, you can override any of the settings that are
+defined in `speedtest_worker.js` using the file
+[`settings.json`](./settings.json). See
+[`speedtest_worker.js`](../speedtest_worker.js) for documentation on the
+different settings (scroll down a bit to where you find the definition of the
+`settings` object).
+
+## Features
+
+- Shows upload and download speed and progress
+- Shows ping and jitter
+- Shows your IP address and internet service provider if the testing server
+  supports it
+- Can handle a single testing server or a list of servers
+- Can save telemetry and share results if the hosting server supports it (set
+  `telemetry_level` in [`settings.json`](./settings.json))
+- Does not require any build steps; implementation is pure JS & CSS
+- Has zero dependencies
+
+## Limitations
+
+- This frontend relies heavily on modern browser features. It should work very
+  well in all modern ("evergreen") browsers, but has **no** backwards compatibility
+  with older browsers.
+
+## Credits
+
+**Design** by fromScratch Studio - 2022, 2023
+([www.fromscratch.io](https://www.fromscratch.io))
+
+> "During Hacktoberfest 2022 & 2023, fromScratch Studio took on the request for
+> LibreSpeed redesign and UI improvements. We ran 2 design sprints one on '22 and
+> one on '23, and produced high-fidelity screens for LibreSpeed redesign.
+> Furthermore, this year, we produced high-fidelity screens for mobile-view as
+> well."
+
+_-- Chris-ZoGo, https://github.com/librespeed/speedtest/issues/585_
+
+**Implementation** by Timendus - 2024
+([https://github.com/Timendus](https://github.com/Timendus))
+
+> "I had a couple of days of free time, and I came across the design by
+> fromScratch. I thought it looked great, and that it deserved a good
+> implementation, so I set out to make one. I've taken the liberty of changing a
+> couple of details, coming up with some animations and adding a few small
+> features, but otherwise I've tried to stay as close to the "intention" of the
+> design as I could."
+
+_-- Timendus, https://github.com/librespeed/speedtest/pull/649_