sensors: add readme with examples

sensors/README.md

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+# Sensor manager
+
+Module provides a basic interface for a variety of sensors.
+
+Assumptions:
+ - the client communicates with manager via messages to **/dev/sensors**,
+ - there is an external library **libsensors** containing sensor structures and message events - external API for the client
+ - sensors server takes sensors description as arguments list:  <sensor_name:bus:others>; arguments might be specific for the sensor driver,
+ - each sensor registers itself in a sensor manager via constructor,
+
+# Examples
+
+
+## Server usage
+``` sh
+sensors -s lsm9dsxx:/dev/spi1.0:argv -s lps25xx:/dev/i2c0`
+```
+
+## Client app:
+
+```c
+#include <stdio.h>
+#include <stdlib.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <sys/ioctl.h>
+
+#include <libsensors.h>
+
+int main(int argc, char **argv)
+{
+	int fd;
+	sensors_data_t *data;
+	unsigned char buff[0x400];
+	sensor_type_t types;
+	sensors_ops_t ops = {0};
+
+	fd = open("/dev/sensors", O_RDWR);
+	if (fd < 0) {
+		return EXIT_FAILURE;
+	}
+
+	/* Get available sensor types */
+	ioctl(fd, SMIOC_SENSORSAVAIL, &types);
+
+	/* Define sensor types request */
+	ops.types = (types & SENSOR_TYPE_BARO) | (types & SENSOR_TYPE_ACCEL) | (types & SENSOR_TYPE_GYRO);
+	ioctl(fd, SMIOC_SENSORSSET, &ops);
+
+	if ((ops.evtSz * sizeof(sensor_event_t) + sizeof(((sensors_data_t *)0)->size)) > sizeof(buff)) {
+		fprintf(stderr, "Buff is too small\n");
+		return EXIT_FAILURE;
+	}
+
+	for (int i = 0; i < 15; ++i) {
+		sleep(1);
+		read(fd, buff, 0x600);
+
+		data = (sensors_data_t *)buff;
+		printf("Sz: %d\n", data->size);
+
+		for (int j = 0; j < data->size; ++j) {
+			switch (data->events[j].type) {
+			case SENSOR_TYPE_ACCEL:
+				printf("Accel, timestamp: %llu\n", data->events[j].timestamp);
+				printf("acce - x: %u, y: %u, z: %u\n", data->events[j].accels.accelX, data->events[j].accels.accelY, data->events[j].accels.accelZ);
+				break;
+
+			case SENSOR_TYPE_BARO:
+				printf("Baro, timestamp: %llu\n", data->events[j].timestamp);
+				printf("baro - pressure: %u\n", data->events[j].baro.pressure);
+				break;
+
+			case SENSOR_TYPE_GYRO:
+				printf("Gyro, timestamp: %llu\n", data->events[j].timestamp);
+				printf("gyro - x: %u, y: %u, z: %u\n", data->events[j].gyro.gyroX, data->events[j].gyro.gyroY, data->events[j].gyro.gyroZ);
+				break;
+
+			default:
+				break;
+			}
+		}
+	}
+
+	close(fd);
+
+	return EXIT_SUCCESS;
+}
+```
+</details>