several fixes and krontab migration

docs/krontab/introduction/faq.md

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+# FAQ
+
+#### How oftern new versions are releasing?
+
+Not very often. It depend on libraries (coroutines, korlibs/klock) updates and on some new awesome, but lightweight, features coming.
+
+#### Where this library could be useful?
+
+First of all, this library will be useful for long uptime applications which have some tasks to do from time to time.
+
+#### How to use crontab-like syntax?
+
+In two words, you should call `buildSchedule` or `createSimpleScheduler`:
+
+```kotlin
+buildSchedule("5 * * * *").asFlow().collect { /* do something */ }
+```
+
+You can read more about syntax in [String format](../describing/string-format.md) section.

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

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+# 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`

docs/krontab/introduction/including-in-project.md

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+# Including in project
+
+In two words, you must add dependency `dev.inmo:krontab:$krontab_version` to your project. The latest version presented by next badge:
+
+ [![Maven Central](https://maven-badges.herokuapp.com/maven-central/dev.inmo/krontab/badge.svg)](https://maven-badges.herokuapp.com/maven-central/dev.inmo/krontab)
+
+### Notice about repository
+  
+To use this library, you will need to include `MavenCentral` repository in you project
+
+###### build.gradle
+
+```groovy
+mavenCentral()
+```
+
+### Dependencies
+
+Next snippets must be placed into your `dependencies` part of `build.gradle` (for gradle) or `pom.xml` (for maven).
+
+#### Gradle
+
+```groovy
+implementation "dev.inmo:krontab:$krontab_version"
+```
+
+#### Maven
+
+```xml
+<dependency>
+    <groupId>dev.inmo</groupId>
+    <artifactId>krontab</artifactId>
+    <version>${krontab_version}</version>
+</dependency>
+```