Make programmers.txt snippet compatible with platforms that use pluggable discovery syntax (#171)

A new flexible and powerful "pluggable discovery" system was added to the Arduino boards platform framework. This system
makes it easy for Arduino boards platform authors to use any arbitrary communication channel between the board and
development tools.

Boards platform configurations that use the old property syntax are automatically translated to the new syntax by
Arduino CLI.

https://arduino.github.io/arduino-cli/latest/platform-specification/#sketch-upload-configuration

This translation is only done in platforms that use the old syntax exclusively. If `pluggable_discovery` properties are
defined for the platform then the new pluggable discovery-style `program.tool` properties must be defined as well.

Since the programmers.txt snippet provided in the readme is intended to be used with any platform, it must be compatible
with those that use the new pluggable discovery syntax. This is accomplished by adding an
`arduinoOTA115.program.tool.default` property.

It is also important to provide compatibility with older versions of Arduino development tools. For this reason, the old
style property `arduinoOTA115.program.tool` is retained. Old versions of the development tools will treat the `arduinoOTA115.program.tool.default`
property as an arbitrary user defined property with no special significance and the new versions of the development
tools will do the same for the `arduinoOTA115.program.tool` property.

Author: per1234

README.md

@@ -75,6 +75,7 @@ Example OTA 'programmer' configuration in programmers.txt:
 ```
 arduinoOTA104.name=Arduino OTA (192.168.1.104)
 arduinoOTA104.program.tool=arduinoOTA
+arduinoOTA104.program.default=arduinoOTA
 arduinoOTA104.ip=192.168.1.104
 ```