docs: update README.md with matrix config

Dockerfile

@@ -6,6 +6,6 @@ COPY requirements.txt /app
 RUN pip install -r requirements.txt
 
 COPY stockbot-buyvm.py /app
-COPY config.py /app
+COPY config.json /app
 
 CMD ["python", "stockbot-buyvm.py"]

README.md

@@ -2,9 +2,16 @@
 Send alerts when [BuyVM](https://buyvm.net) has KVM slices in stock.
 
 ## Usage
-1. Create a Discord Webhook and add it to `config.py`:
-```python
-DISCORD_WEBHOOK = '<discord webhook url>'
+1. Create a JSON configuration file in `config.json`:
+```json
+{
+  "matrix": {
+    "homeserver": "https://matrix.juggalol.com",
+    "username": "",
+    "password": "",
+    "room_id": ""
+  }
+}
 ```
 
 2. Build Docker container: