boards/doc: Add a guide to find physical board pins

This is needed for those who do not know about searching the periph conf
It gives some hints on how to physically locate you board if connecting up sensors

boards/doc.txt

@@ -15,4 +15,57 @@
  * configuration of a controller and some external devices such as sensors or
  * radios. All aspects concerning configuration of GPIO pins, MCU clock and
  * device drivers should go into this module.
+ *
+
+Guide to board pinouts {#pinout_guide}
+=====================================
+
+The following are the steps to take to locate specific pinouts on boards.
+This is useful when connecting external hardware to RIOT supported boards.
+
+1. Check the [board documentation](http://doc.riot-os.org/group__boards.html)
+to see if the pinout is available
+2. If the pinout is not available or up to date check the following:
+    - `boards/<BOARD>/include/periph_conf.h` for board specific
+peripherals such as UART or SPI pins
+    - `boards/<BOARD>/include/board.h` for GPIO specific pins such
+as `LED0` or `BTN0`
+    - `boards/common/<COMMON_BOARD_OR_CPU>/include/periph_conf_common.h`
+for shared pins that are the same for many different boards (board_common.h
+or others will be available too)
+    - `boards/common/<COMMON_BOARD_OR_CPU>/include/<PERIPH_DECLARATION>`
+also may contain specific periphirals such as
+`PERIPH_DECLARATION = cfg_i2c1_pb8_pb9.h` which indicates I2C pins are on
+pb8 and pb9
+3. Once the needed pins are determined with either a port where
+`PORTA = PORT_A = PA = 0` or `PORTB = PORT_B = PB = 1` and pin
+number, such as `PA, 1`, correlate that to the board (the board
+may either have the designators or search online to find which GPIO
+corresponds to physical location on the board)
+4. Match the `GPIO` with the specific pin and connect the wires
+
+### Examples of finding pinouts
+The following will be different examples of how to find pinouts
+
+#### Finding pins of the I2C device 0 on samr21-xpro
+1. The documentation says I2C is supported but does not say the pinout
+2. Checking the pinout in `boards/samr21-xpro/include/periph_conf.h` the
+`scl_pin = GPIO_PIN(PA, 17)` and the `sda_pin = GPIO_PIN(PA, 16)`
+3. Since the board has the GPIO port and pin printed on the board the
+physical location is `PA16` and `PA17`
+
+#### Finding pins of the UART device 1 on nucleo-f103rb
+_The dev number is only the location in the array and does not mean the
+number of the peripheral_
+1. The documentation says UART is supported but does not say the pinout
+2. Checking the pinout in `boards/nucleo-f103rb/include/periph_conf.h` the
+`rx_pin = GPIO_PIN(PORT_A, 10)`, the `tx_pin = GPIO_PIN(PORT_A, 9)` and the
+`dev = USART1`, though device 1 can equal any USARTn number
+3. Since the board does not have information on where these pins are
+physically located it is recommended to search
+[online](https://os.mbed.com/platforms/ST-Nucleo-F103RB/) for that board
+4. After locating the physical layout the rx_pin can be found on CN9-1
+using either `PA_10` or `UART1_RX` and the tx_pin can be found on CN5-1
+using either `PA_9` or `UART1_TX`, it is recommended to use the GPIO pin as
+sometimes the peripheral may be using an alternate pin
  */