Fix the unusable programming port by making the erase time longer (250ms->500ms). 
According to the datasheet, the recommended erase time is greater than 220ms. In most cases the original program should be OK. However, in my case, the uploading is almost 90% fail rate (Device not found at comx port...). But I can manually press the erase button before the uploading, and it works well. After the proposed changes, the programming port works as expected and I no longer necessary to hit the erase button.

Fix some mistakes and add some information to make it easier for future developments.

Update readme.txt

Update Arduino-usbserial.c

make the erase time longer to fix the unusable programming port for some board

Make the erase time longer to fix the unusable programming port for some Arduino Due boards.