Add JTAG pin specification to README.md

[0xjmux]

I'm designing an adapter and wanted a JTAG pinout for the SOICbite, and figured I might as well contribute it upstream.

This PR just adds the JTAG column to the table, as below:

Pin	UART	SPI	SWIM	SWD	ESP8266	USB	I2C	PIC ICSP	JTAG
1	Vcc	Vcc	Vcc	Vcc	Vcc	Vcc	Vcc	Vcc	Vcc
2	RX2	CS			GPIO0	D-		Vpp	TMS
3	TX2				GPIO2	D+			TCK
4	GND	GND	GND	GND	GND	GND	GND	GND	GND
5	RX	MOSI/MOMI	SWIM	SWDIO	RX		SDA	DAT	TDI
6	TX	MISO		SWO	TX			AUX	TDO
7	CTS	SCK/CLK		SWCLK	CH_PD		SCL	CLK	RTCK
8	RTS	RST	NRST	NRST	RST	ID			NRST