This step wasn't included, which will lead to errors when TinyGo is
built using `go install`.

In 4 subsections:

1) Writing to the serial port using fmt.Print(), fmt.Println(), and the
   built-in functions print() and println(). Explain flash memory consumption
   of `fmt` package.
2) Running `tinygo monitor` to see the output on the host computer.
3) Reading from the serial port.
4) Alternative serial monitors (Arduino IDE, pyserial, picocom).

Address review comments:
* Add COM1 and COM31 for Windows
* Add `tinygo monitor -baudrate=9600` for AVR processors
* Fix couple of grammar typos.
* Add explanation of "baud rate".
* Add instructions to use `flash -x` flag to see the port of the
  microcontroller.

Expanded the 'Serial Monitor on Host Computer` section into multiple
subsections:

* monitor subcommand
* -monitor flag
* Baud Rate
* Serial Port on Host
* Multiple Microcontrollers

The 'Serial Monitor on Host Computer' section is now LOT longer than I had
originally intended, I had intended to only give an entry-level usage of
`tinygo monitor` just to get a newcomer started on serial monitors. But I guess
this is the only place where the 'tinygo monitor' subcommand is explained, so
we are tempted to describe all of its conveniences.

Moved some material in 'Alternative Serial Monitors' section into the above
subsections for better flow.

This is necessary for WebAssembly.

I totally forgot about this one! So here it is.

reference/usage/subcommands: Document the `tinygo monitor` subcommand. And the `-monitor` flag on the
`tinygo flash` command.

Signed-off-by: John Clark [email protected]