Added definition/documentation for pressure sensors

Change-Id: I68b1bb74ebf1aceee09b6e8cbb8e37b906dbdcd0
diff --git a/include/hardware/sensors.h b/include/hardware/sensors.h
index 2bbc8c9..0a3d673 100644
--- a/include/hardware/sensors.h
+++ b/include/hardware/sensors.h
@@ -205,6 +205,13 @@
  * Light sensors report a value only when it changes and each time the
  * sensor is enabled. setDelay() is ignored.
  *
+ * Pressure
+ * --------
+ *
+ * The pressure sensor value is returned in hectopascal (hPa)
+ *
+ * Pressure sensors report events at a constant rate defined by setDelay().
+ *
  */
 typedef struct {
     union {
@@ -264,6 +271,9 @@
 
         /* light in SI lux units */
         float           light;
+
+        /* pressure in hectopascal (hPa) */
+        float           pressure;
     };
     uint32_t        reserved1[4];
 } sensors_event_t;