add way to export Discord server

Signed-off-by: Jakub Sokołowski <jakub@status.im>

discord/README.md

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+# Description
+
+This script is an example of how a Discord Server can be exported in its entirety.
+
+This script is using the `tyrrrz/discordchatexporter` Docker image of [DiscordChatExporter](https://github.com/Tyrrrz/DiscordChatExporter).
+
+# Usage
+
+```sh
+export API_TOKEN=abcd
+export GUILD_ID=1234
+./export.sh
+```
+
+# Instructions
+
+You can get instructions on getting the necessary values by running:
+```sh
+docker run --rm -it tyrrrz/discordchatexporter guide
+```

discord/export.sh

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+#!/usr/bin/env bash
+
+set -e
+
+if [[ -z "$API_TOKEN" ]]; then
+    echo "The API_TOKEN env variable is not set"
+    exit 1
+fi
+
+if [[ -z "$GUILD_ID" ]]; then
+    echo "The GUILD_ID env variable is not set"
+    exit 1
+fi
+
+docker run --rm -it -v "$PWD:/export" \
+    tyrrrz/discordchatexporter \
+    exportguild --bot \
+        --token "$API_TOKEN" \
+        --guild "$GUILD_ID" \
+        --output "/export" \
+        --format HtmlDark