Updated docs

Author: cmaglie

docs/pluggable-discovery-specification.md

@@ -377,3 +377,67 @@ myboard.upload_port.1.apples=40
 
 will match on both `pears=20, apples=30` and `pears=30, apples=40` but not `pears=20, apples=40`, in that sense each
 "set" of identification properties is independent from each other and cannot be mixed for port matching.
+
+#### Identification of board options
+
+The board identification can also identify [custom board options](platform-specification.md#custom-board-options) if the
+corresponding `upload_port` properties are defined:
+
+```
+BOARDNAME.menu.MENUNAME.MENUOPTION.upload_port.IDENTIFIER=Value
+BOARDNAME.menu.MENUNAME.MENUOPTION.upload_port.#.IDENTIFIER=Value
+```
+
+a discovery tools capable of detecting which menu options where used could report properties to match those lines. Let's
+see an example to clarify it, in the following `boards.txt`:
+
+```
+myboard.upload_port.pid=0x0010
+myboard.upload_port.vid=0x2341
+myboard.menu.cpu.atmega1280=ATmega1280
+myboard.menu.cpu.atmega1280.upload_port.c=atmega1280          <--- identification property for cpu=atmega1280
+myboard.menu.cpu.atmega1280.build_cpu=atmega1280
+myboard.menu.cpu.atmega2560=ATmega2560
+myboard.menu.cpu.atmega2560.upload_port.c=atmega2560          <--- identification property for cpu=atmega2560
+myboard.menu.cpu.atmega2560.build_cpu=atmega2560
+myboard.menu.mem.1k=1KB
+myboard.menu.mem.1k.upload_port.mem=1                         <--- identification property for mem=1k
+myboard.menu.mem.1k.build_mem=1024
+myboard.menu.mem.2k=2KB
+myboard.menu.mem.2k.upload_port.1.mem=2                       <------ identification property for mem=2k (case 1)
+myboard.menu.mem.2k.upload_port.2.ab=ef                       <---\
+myboard.menu.mem.2k.upload_port.2.cd=gh                       <---+-- identification property for mem=2k (case 2)
+myboard.menu.mem.2k.build_mem=2048
+```
+
+we have a board called `myboard` with two custom menu options `cpu` and `mem`. The discovery may provide extra
+identification properties to determine the custom menu options of the connected board, for example the following
+properties:
+
+```
+vid=0x0010
+pid=0x2341
+c=atmega2560
+```
+
+will match the FQBN `mypackage:avr:myboard:cpu=atmega2560` becuase of the property `c=atmega2560`. The identification
+properties:
+
+```
+vid=0x0010
+pid=0x2341
+c=atmega2560
+mem=2
+```
+
+will match the FQBN `mypackage:avr:myboard:cpu=atmega2560,mem=2k`. The identification properties:
+
+```
+vid=0x0010
+pid=0x2341
+c=atmega2560
+ab=ef
+cd=gh
+```
+
+will match the FQBN `mypackage:avr:myboard:cpu=atmega2560,mem=2k` too (they will match the second set for `mem=2k`).