Added code to ensure that code snippets in Readme compile. (#46)

* Update README.md

Added missing stream insertion operator to example.   Also, header files for chrono and thread as well as a missing namespace for literals added.

---------

Co-authored-by: Per Malmberg <PerMalmberg@users.noreply.github.com>

README.md

@@ -6,10 +6,16 @@ A C++ scheduling library using cron formatting.
 Libcron offers an easy to use API to add callbacks with corresponding cron-formatted strings:
 
 ```
+#include <libcron/Cron.h>
+#include <libcron/CronData.h>
+#include <chrono>
+#include <thread>
+using namespace std::chrono_literals;
+
 libcron::Cron cron;
 
 cron.add_schedule("Hello from Cron", "* * * * * ?", [=](auto&) {
-	std::cout << "Hello from libcron!" std::endl;
+	std::cout << "Hello from libcron!" << std::endl;
 });
 ```
 
@@ -19,7 +25,7 @@ To trigger the execution of callbacks, one must call `libcron::Cron::tick` at le
 while(true)
 {
 	cron.tick();
-	std::this_thread::sleep_for(500mS);
+	std::this_thread::sleep_for(500ms);
 }
 ```
 
@@ -188,11 +194,16 @@ the '?'-character to ensure that it is not possible to specify a statement which
 |Expression | Meaning
 | --- | --- |
 | * * * * * ? | Every second
+| 0 * * * * ? | Every minute
 | 0 0 12 * * MON-FRI | Every Weekday at noon
 | 0 0 12 1/2 * ?	| Every 2 days, starting on the 1st at noon
 | 0 0 */12 ? * * | Every twelve hours
 | @hourly | Every hour
 
+Note that the expression formatting has a part for seconds and the day of week. 
+For the day of week part, a question mark ? is utilized. This format
+may not be parsed by all online crontab calculators or expression generators.
+
 ## Convenience scheduling
 
 These special time specification tokens which replace the 5 initial time and date fields, and are prefixed with the '@' character, are supported:

test/CronScheduleTest.cpp

@@ -184,6 +184,15 @@ SCENARIO("Examples from README.md")
                          DT(2018_y / 03 / 1, hours{12}, minutes{13}, seconds{48})
                  }));
 
+    REQUIRE(test("0 * * * * ?", DT(2018_y / 03 / 1, hours{ 12 }, minutes{ 0 }, seconds{ 10 }),
+        {
+                DT(2018_y / 03 / 1, hours{12}, minutes{1}, seconds{0}),
+                DT(2018_y / 03 / 1, hours{12}, minutes{2}, seconds{0}),
+                DT(2018_y / 03 / 1, hours{12}, minutes{3}, seconds{0}),
+                DT(2018_y / 03 / 1, hours{12}, minutes{4}, seconds{0})
+        }));
+
+
     REQUIRE(test("0 0 12 * * MON-FRI", DT(2018_y / 03 / 10, hours{12}, minutes{13}, seconds{45}),
                  {
                          DT(2018_y / 03 / 12, hours{12}),
@@ -212,4 +221,4 @@ SCENARIO("Examples from README.md")
 SCENARIO("Unable to calculate time point")
 {
     REQUIRE_FALSE(test( "0 0 * 31 FEB *", DT(2021_y / 1 / 1), DT(2022_y / 1 / 1)));
-}
+}