adding SPI support

docs/reference/pins/spi-write.md

@@ -0,0 +1,19 @@
+# SPI Write
+
+Write to the SPI Slave and return the response.
+
+```sig
+pins.spiWrite(0);
+```
+
+### Parameters
+
+* ``value``: value	Data to be sent to the SPI slave
+
+### Returns
+
+* a [number](/reference/types/number) Response from the SPI slave
+
+### See also
+
+[SPI](https://developer.mbed.org/handbook/SPI)