[docs] read and burstRead methods missing in I2C Web IDL

[haoxli]

There are read and burstRead methods in I2C API, but not defined in Web IDL:
void read(octet device, unsigned int size, octet registerAddress);
void burstRead(octet device, unsigned int size, octet registerAddress);

And I think the read method should return a value, shouldn't be a void method.

[brianjjones]

Fixed with #583

[xiaolongx]

Add read and burstRead methods in i2c.md with commit #583