docs: add README & screenshot

README.md

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+# video-call-over-email
+
+<!-- Yep, actual video call over actual email. -->
+
+A [webxdc](https://webxdc.org) app for actual video calls over actual email (albeit with 15-second ping (but [it may improve soon](https://github.com/deltachat/deltachat-core-rust/pull/4904))).
+
+![A simplistic video call app UI](./screenshot.jpg)
+
+This is just a prototype.
+
+## Usage
+
+As was said, this is a [webxdc](https://webxdc.org) app. It requires a webxdc-supporting messenger to run. Delta Chat is one such messenger.
+
+However, as of 2023-11-16, sending audio/video won't work on unmodified versions of Delta Chat, but [this may change](https://support.delta.chat/t/allow-access-to-camera-geolocation-other-web-apis/2446?u=wofwca) in the near future.
+
+_Receiving_ video does work on regular Delta Chat.
+
+### Modifying Delta Chat
+
+Below are instructions on how to modify Delta Chat. But make sure **not to launch any webxdc apps** that you don't trust on the modified version of Delta Chat as it is **insecure**. Tested on Delta Chat 1.40.4.
+
+1. Download [Delta Chat Desktop](https://delta.chat/).
+2. Find the `DeltaChat/resources/app.asar` file in the app folder.
+3. Open it as a ZIP file.
+4. Open the `tsc-dist/main/deltachat/webxdc.js` file inside the archive.
+5. Find the line
+
+    ```javascript
+    const logPermissionRequest = (permission) => {
+    ```
+
+    and add a line
+
+    ```javascript
+    return true; // ADDED BY ME
+    ```
+
+    right below it.
+6. Save the modified app.asar file.
+7. After you're done playing around with this app, make sure to remove the line you added, or simply reinstall Delta Chat.
+
+### Running the app
+
+1. Launch Delta Chat.
+2. Build an `.xdc` file with `./create-xdc.sh`, or just [download it from the "Releases" section](https://github.com/WofWca/video-call-over-email/releases/latest/download/webxdc-video-call.xdc).
+3. Send the `.xdc` file to a chat.
+4. Wait for some other chat members to launch the app.
+5. Press "Start sending my media"; or just wait for others to send theirs.
+
+Keep in mind that video data takes a lot of space. Make sure not to waste the storage quota on your email server. The expected bitrate in this app for audio + video is ~50 MB / hour per member and ~2 MB / hour per member for just audio.
+
+## Why
+
+Because it's funny.
+
+And it might actually become an actually useful video call app, when:
+
+- [the ratelimit](https://github.com/deltachat/deltachat-core-rust/blob/212751f173139aab3daadcd77388b3551004cabe/src/context.rs#L382) gets much better than 1 email per 10 seconds
+- webxdc apps can be [allowed camera permission](https://support.delta.chat/t/allow-access-to-camera-geolocation-other-web-apis/2446?u=wofwca)
+- A way is found to not fill up email servers with audio/video data (maybe something like "ephemeral webxdc messages")
+
+## How it works
+
+Nope, it's not WebRTC.
+
+1. Record 10 seconds of your camera stream with a [`MediaRecorder`](https://developer.mozilla.org/en-US/docs/Web/API/MediaStream_Recording_API).
+2. Serialize the data.
+3. Send it over email (with [`webxdc.sendUpdate()`](https://docs.webxdc.org/spec/sendUpdate.html)).
+4. Repeat from step 1.
+
+When we receive data, deserialize it and display it using [Media Source Extensions API](https://developer.mozilla.org/en-US/docs/Web/API/Media_Source_Extensions_API).
+
+## Web demo (no email involved)
+
+If you just want to see how the app feels, without actually communicating with anyone using email, go to <https://wofwca.github.io/video-call-over-email/> .