fix(readme): suppress direnv loading output in cron

By default direnv outputs this while executing:
```
direnv: loading ~/python-itho-wpu/.envrc
```

In a cronjob this results in emails, or when no MTA is configured:
```
CRON[1095]: (CRON) info (No MTA installed, discarding output)
```

README.md

@@ -21,6 +21,7 @@ See the [pislave](https://github.com/ootjersb/pislave#wiring) project
        pigpiod \          # Pigpio daemon to communicate over the I2C bus
        python3-pigpio \   # Python module to talk to the pigpio daemon
        python3-pyodbc \   # Python module to access an ODBC database
+       odbcinst1debian2 \ # Library for accessing ODBC config files (odbc-mdbtools dependency)
        sqlite3 \          # To store a subset of the Itho database in SQLite
        direnv \           # Environment variable manager to store credentials for InfluxDB
        python3-influxdb   # To export measurements to InfluxDB
@@ -129,7 +130,7 @@ Assuming InfluxDB is running on the Raspberry Pi as well.
    cat > /etc/cron.d/itho <<EOT
    PATH=/usr/bin:/bin:/usr/sbin:/sbin
 
-   */5 * * * * root cd /root/python-itho-wpu && direnv exec . ./itho-wpu.py --action getdatalog --export-to-influxdb >> /var/log/itho.log
+   */5 * * * * root cd /root/python-itho-wpu && DIRENV_LOG_FORMAT='' direnv exec . ./itho-wpu.py --action getdatalog --export-to-influxdb >> /var/log/itho.log
    EOT
    ```