several fixes and krontab migration

docs/krontab/introduction/how-to-use.md

@@ -0,0 +1,57 @@
+# How to use
+
+## Previous pages
+
+* [Including in project](including-in-project.md)
+
+## `buildSchedule`
+
+> NOTE: **Custom KronScheduler**
+> You always able to create your own scheduler. In this section will be presented different ways and examples around standard `CronDateTimeScheduler` builders `buildSchedule`. You can read about schedulers in [KrontabScheduler](../describing/krontabscheduler.md)
+
+Currently, `buildSchedule` is the recommended start point for every scheduler. Usually, it is look like:
+
+```kotlin
+val scheduler = buildSchedule("5 * * * *")
+```
+
+Or:
+
+```kotlin
+val scheduler = buildSchedule {
+  seconds {
+    at(5)
+  }
+}
+```
+
+On the top of any `KronScheduler` currently there are several groups of extensions:
+
+* Executes
+* Shortcuts
+* Flows
+
+### Executes
+
+All executes are look like `do...`. All executes are described below:
+
+* `doOnce` - will get the next time for executing, delay until that time and call `block` with returning of the `block` result
+* `doWhile` - will call `doOnce` while it will return `true` (that means that `block` must return `true` if it expects that next call must happen). In two words: it will run while `block` returning `true`
+* `doInfinity` - will call the `block` using `doWhile` with predefined returning `true`. In two words: it will call `block` while it do not throw error
+
+### Shortcuts
+
+Shortcuts are the constants that are initializing in a lazy way to provide preset `KronScheduler`s. For more info about `KrontabScheduler` you can read its own [page](../describing/krontabscheduler.md).
+
+* `AnyTimeScheduler` - will always return incoming `DateTime` as next
+* `Every*Scheduler` - return near * since the passed `relatively`:
+  * `EverySecondScheduler`
+  * `EveryMinuteScheduler`
+  * `EveryHourScheduler`
+  * `EveryDayOfMonthScheduler`
+  * `EveryMonthScheduler`
+  * `EveryYearScheduler`
+
+### Flows
+
+Here currently there is only one extension for `KronScheduler`: `KronScheduler#asFlow`. As a result you will get `Flow<DateTime>` (in fact `SchedulerFlow`) which will trigger next `emit` on each not null `next` `DateTime`