Update documentation for BOARD_SPEED.

HISTORY.md

@@ -5,6 +5,7 @@ The following is the rough list of changes that went into different versions.
 I tried to give credit whenever possible. If I have missed anyone, kindly add it to the list.
 
 ### In Development
+- Tweak: Add support for BOARD_SPEED for board.menu.speed entries in boards.txt files. (https://github.com/dewhisna)
 - Fix: Moved CORE_LIB to the last position of the defined linked objects. (https://github.com/wingunder)
 - Fix: Moved ATtiny examples to ATtinyBlink, updated alternate core instructions (issue #537) (https://github.com/sej7278)
 - Fix: Add -fno-devirtualize flag to workaround g++ segfault bug (issue #486). (https://github.com/sej7278)

arduino-mk-vars.md

@@ -344,6 +344,24 @@ BOARD_SUB=atmega168
 
 ----
 
+### BOARD_SPEED
+
+**Description:**
+
+Allow selection of f_cpu value specified in `boards.txt` as `board_tag.menu.speed.board_speed`.
+For example, the Watterott ATmega328PB library [https://github.com/watterott/ATmega328PB-Testing](https://github.com/watterott/ATmega328PB-Testing).
+
+**Example:**
+
+```Makefile
+# Select 20MHz clock
+BOARD_SPEED=20mhz
+```
+
+**Requirement:** *Optional to override main board f_cpu setting*
+
+----
+
 ### MONITOR_PORT
 
 **Description:**