Clarify docs to make compiling/flashing easier for newcomers

[flaki]

Getting the T2 into bootloader mode

Issue:
It was quite hard to get the T2 into bootloader mode. For the first two tries I didn't manage, and didn't know (for someone who sees a Tessel for the first time, it wouldn't be straightforward what he should see)

Fix:
Explain the signs of the T2 entering bootloader mode (eg.: blue/power led blinking / red led on board continously lit)

Nit: alarming error message following flashing

Issue:

$ dfu-util -aFlash -d 1d50:6097 -D build/firmware.bin
dfu-util 0.8
...

Copying data from PC to DFU device
Download    [=========================] 100%        12624 bytes
Download done.
state(7) = dfuMANIFEST, status(0) = No error condition is present
dfu-util: unable to read DFU status after completion

Fix:
Would be nice if we could get rid of the unable to read DFU status after completion-message?