create library and make two examples of usage

I2Csensors/examples/Readme.md

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+# Examples of library usage
+
+### logmany.c
+
+Creates executable `log`. The purpose of this is to show how you can use library wit lots of sensors (even with fully similar)
+divided by groups (to prevent same addresses on one bus) switching by PCA9548A.
+
+Usage:
+```
+  -H, --hlog=arg       humidity logging file
+  -P, --plog=arg       pressure logging file
+  -T, --tlog=arg       temperature logging file
+  -a, --muladdr=arg    multiplexer I2C address
+  -d, --device=arg     I2C device path
+  -h, --help           show this help
+  -i, --interval=arg   logging interval, seconds (default: 10)
+  -m, --presmm         pressure in mmHg instead of hPa
+```
+
+### single_sensor.c
+
+Creates executable `single`. Open single sensor and show its parameters.
+
+Usage:
+
+```
+  -H, --heater=arg    turn on/off heater (if present)
+  -a, --address=arg   sensor's address (if not default)
+  -d, --device=arg    I2C device path
+  -h, --help          show this help
+  -l, --list          list all supported sensors
+  -m, --presmm        pressure in mmHg instead of hPa
+  -s, --sensor=arg    sensor's name
+
+```