refactor(sntp): add lwip sntp example

examples/protocols/sntp/README.md

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+# Example: using LwIP SNTP module and time functions
+
+This example demonstrates the use of LwIP SNTP module to obtain time from Internet servers. See the README.md file in the upper level 'examples' directory for more information about examples.
+
+## Obtaining time using LwIP SNTP module
+
+ESP8266 connects to WiFi and obtains time using SNTP.
+See `initialize_sntp` function for details.
+
+## Working with time
+
+To get current time, [`gettimeofday`](http://man7.org/linux/man-pages/man2/gettimeofday.2.html) function may be used. Additionally the following [standard C library functions](https://en.cppreference.com/w/cpp/header/ctime) can be used to obtain time and manipulate it:
+
+	gettimeofday
+	time
+	asctime
+	clock
+	ctime
+	difftime
+	gmtime
+	localtime
+	mktime
+	strftime
+
+To set time, [`settimeofday`](http://man7.org/linux/man-pages/man2/settimeofday.2.html) POSIX function can be used. It is used internally in LwIP SNTP library to set current time when response from NTP server is received.
+
+## Timezones
+
+To set local timezone, use [`setenv`](http://man7.org/linux/man-pages/man3/setenv.3.html) and [`tzset`](http://man7.org/linux/man-pages/man3/tzset.3.html) POSIX functions. First, call `setenv` to set `TZ` environment variable to the correct value depending on device location. Format of the time string is described in [libc documentation](https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html). Next, call `tzset` to update C library runtime data for the new time zone. Once these steps are done, `localtime` function will return correct local time, taking time zone offset and daylight saving time into account.