docs: add readme listing environment variables

README.md

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+# Relay
+
+## Installation
+
+### Environment Variables
+
+|Variable|Description|Required|Default Value|
+|-|-|:-:|-|
+|`HOST`|The application host.|✔||
+|`PORT`|The application port.|✔||
+|`PUBLIC_URL`|The public endpoint of the web ui.|✔||
+|`BOT_TOKEN`|Authentication for the Discord Bot.|✔||
+|`DATABASE_URL`|Postgresql connection string.|✔||
+|`OAUTH_URL`|Discord authentication URL.|✔||
+|`CALLBACK_URL`|Discord authentication callback URL.|✔||
+|`CLIENT_ID`|Discord application client ID.|✔||
+|`CLIENT_SECRET`|Discord application client secret.|✔||
+|`DISCORD_USER_IDS`|CSV of Discord User IDs allowed to access the web ui.|✔||