Make update instructions clearer

[Frijol]

Within #compiling section:

to make sure T2 is detected:

➜  dfu-util --list
...
Found DFU: [1209:7551] ver=0002, devnum=13, cfg=1, intf=0, alt=1, name="SRAM", serial="UNKNOWN"
Found DFU: [1209:7551] ver=0002, devnum=13, cfg=1, intf=0, alt=0, name="Flash", serial="UNKNOWN"
Note the vendor id and product id within the brackets (1209:7551 in this case). You'll need to substitute those numbers in the command below to flash the device:

What are we looking for?

[kevinmehall]

Tessels that had the bootloader installed before July have the ids from T1 (1d50:6097) instead, and you'd have to use that ID in the following command. We should just track down those dozen Tessels and update their bootloaders and then there's no need to worry about this.