add an info text how to encode free-for-all magic values

Because of pressing interest there will be following free-for-use scheme.
It will always start with 0xf as the 4 most significant bits, and then
can be encoded based of the signature of the AVR, the IO-Port used for USB,
the USB dataline PINs, the external interrupt used for USB, the clock frequency
the BOD voltage and if the watchdog is enabled by fuses.

The magic will be: 0xf??????? in binary 0b1111ZZZZ YYYYYYYY WXXXVUUU TTSSSRRR

--> where the 4 bit of Z correspond to the middle of the 3 byte AVR signature:
	SIGNATURE_1 == 0x90	<-->	ZZZZ == 0b0000
	SIGNATURE_1 == 0x91	<-->	ZZZZ == 0b0001
	SIGNATURE_1 == 0x92	<-->	ZZZZ == 0b0010
	SIGNATURE_1 == 0x93	<-->	ZZZZ == 0b0011
	SIGNATURE_1 == 0x94	<-->	ZZZZ == 0b0100
	SIGNATURE_1 == 0x95	<-->	ZZZZ == 0b0101
	SIGNATURE_1 == 0x96	<-->	ZZZZ == 0b0110
	SIGNATURE_1 == 0x97	<-->	ZZZZ == 0b0111
	SIGNATURE_1 == 0x98	<-->	ZZZZ == 0b1000
	SIGNATURE_1 == 0xA6	<-->	ZZZZ == 0b1001
	SIGNATURE_1 == 0xA7	<-->	ZZZZ == 0b1010 *
	SIGNATURE_1 == 0xA8	<-->	ZZZZ == 0b1011
	reserved			ZZZZ == 0b1100 *
	not free (database magics)	ZZZZ == 0b1101 *
	not free (database magics)	ZZZZ == 0b1110 *
	not free (database magics)	ZZZZ == 0b1111 *
If the AVRs SIGNATURE_1 is different from the presented 12, this coding scheme is
not applicable for you - please ask for a personal magic in this case.

--> where the 8 bit of Y are the SIGNATURE_2 byte (for example 0b00000111 on ATmega8)

--> where the 1 bit of W is watchdog enabled by fuse: 0=no, 1=yes (WD enabled)

--> where the 3 bit of X are the (external) crystal clockspeed
	12MHz external crystal	 <-->	 XXX == 0b000
	12.8MHz external crystal <-->	 XXX == 0b001
	reserved		 <-->	 XXX == 0b010
	15MHz external crystal	 <-->	 XXX == 0b011
	16MHz external crystal	 <-->	 XXX == 0b100
	16.5MHz external crystal <-->	 XXX == 0b101
	18MHz external crystal	 <-->	 XXX == 0b110
	20MHz external crystal	 <-->	 XXX == 0b111
If your clockspeed is different from the presented 8, this coding scheme is
not applicable for you - please ask for a personal magic in this case.

--> where the 1 bit of V depends on the controllers Vcc:
	4.5V (ususally 5V)	 <-->	   V == 0
	3.3V			 <-->	   V == 1

--> where the 3 bit of U define the PORT (USB_CFG_IOPORTNAME) for USB
	PORTA			 <-->	 UUU == 0b000
	PORTB			 <-->	 UUU == 0b001
	PORTC			 <-->	 UUU == 0b010
	PORTD			 <-->	 UUU == 0b011
	PORTE			 <-->	 UUU == 0b100
	PORTF			 <-->	 UUU == 0b101
	PORTG			 <-->	 UUU == 0b110
	PORTH			 <-->	 UUU == 0b111
If your PORT is different from the presented 8:
This coding scheme is more an example. UUU=0b000 really means, the first GPIO port
on the specified MCU. (0b001 is the second, 0b010 the third...)
Normally there aren't more then 8 GPIO ports on an AVR - so this coding works.
If there are more then 8, only the first 8 ports can be encoded and otherwise
this coding scheme is not applicable for you - please ask for a personal magic in this case.

--> where the 2 bit of T define the external interrupt used for USBD
	INT0			 <-->	  TT == 0b00
	INT1			 <-->	  TT == 0b01
	INT2			 <-->	  TT == 0b10
	INT3			 <-->	  TT == 0b11
If you use a different interrupt, this coding scheme is not applicable
for you - please ask for a personal magic in this case.

--> where the 3 bit of S define the PIN (on the previous encoded port) for USB+
    (basically the value of USB_CFG_DPLUS_BIT)
	0			 <-->	 SSS == 0b000
	1			 <-->	 SSS == 0b001
		...
	7			 <-->	 SSS == 0b111

--> where the 3 bit of R define the PIN (on the previous encoded port) for USB-
    (basically the value of USB_CFG_DMINUS_BIT)
	0			 <-->	 RRR == 0b000
	1			 <-->	 RRR == 0b001
		...
	7			 <-->	 RRR == 0b111

WARNING:
	Please note, that this 0xf------- magic does not include
	any information about the bootloader condition pin (and port)!
	This magic just ensures basic protection against different
	layouts. (Which only covers essential data)

	?Maybe as a heuristic?:
	The bootloader condition pin should be the same port as USB,
	at the pin before USB_CFG_DMINUS_BIT.

	If you want to be very safe, request your own magic...

Examples (USB pinning are tinyUSBboard Rev.3 defaults):

"ATmega8@16MHz powered with 5V and no watchdog fused":		0xf3078317
The same just for ATmega328p: 					0xf50f8317

"ATmega1024p @ 20MHz powered with 3V and watchdog fused active":	0xf705fb17
"ATmega2560  @ 12MHz powered with 5V and watchdog fused active":	0xf8010307

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Signed-off-by: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de>