lte-pic32-writer's sendto.txt may disclose URL and the API key

lte-pic32-writer is a writer for PIC32 devices. In versions 0.0.1 and prior, those who use `sendto.txt` are vulnerable to attackers who known the IMEI reading the sendto.txt. The sendto.txt file can contain the SNS(such as slack and zulip) URL and API key. As of time of publication, a patch is not yet available. As workarounds, avoid using `sendto.txt` or use `.htaccess` to block access to `sendto.txt`.