docs(checker): Add documentation

src/services/checker/README.md

+# Service for check availability other services
+
+## Quick Start
+
+```js
+// If you need the single service
+export { CheckerService } from '@pb/services'
+const service = new CheckerService()
+```
+
+or
+
+```js
+// If you need all services
+export { Services } from '@pb/services'
+const services = new Services(PromobotAPI)
+
+services.checkerService
+```
+
+## API
+
+| method             | description                                                |
+| ------------------ | ---------------------------------------------------------- |
+| on                 | Allow subscribe to one of known events                     |
+| registerService    | Allow register service with name                           |
+| checkServiceByName | Allow check registered service                             |
+| checkService       | Allow check any service by instance                        |
+| getAllServices     | Returns array with all registered CheckerItem (see below) |
+| getServiceByName   | Return known CheckerItem (see below)                      |
+
+## API explanation
+
+### CheckerItem interface
+```js
+interface CheckerItem {
+  name: string
+  instance: Service
+  state: 'init' | 'pending' | 'resolve'
+  isActive: boolean
+}
+```
+
+### `on`
+
+The `on` method allow subscribe to one of known events.
+
+| event  | description                     | payload     |
+| ------ | ------------------------------- | ----------- |
+| START  | Emit when check started         | CheckerItem |
+| FINISH | Emit when check finished        | CheckerItem |
+| ERROR  | Emit when an error has occurred | Reason      |
+
+Quick access is also available.
+
+```js
+CheckerService.START
+CheckerService.FINISH
+CheckerService.ERROR
+```
+
+Accordingly.
+
+**Syntax**
+```js
+on (event, callback)
+```
+
+**Parameters**
+1. `event`: (string) - One of known event.
+2. `callback` - Function, given 1 parameter with instance or error.
+
+**Example**
+```js
+const checker = new CheckerService()
+
+checker.registerService('test', {
+  async check () {
+    return Promise.resolve(true)
+  }
+})
+
+checker.on(CheckerService.START, (service) => {
+  console.log(service)
+})
+
+checker.checkServiceByName('test')
+
+// {
+//   name: 'test',
+//   instance: { serviceInstance },
+//   state: 'pending',
+//   isActive: false
+// }
+```
+
+### `registerService`
+
+The `registerService` method allow register service with name as key.
+
+**Syntax**
+```js
+registerService (name, service)
+```
+
+**Parameters**
+1. `name`: (string) - The service name which he will be registered and you will have access by this name.
+2. `service` - A service instance
+
+**Example**
+```js
+const checker = new CheckerService()
+
+const service = {
+  async check () {
+    return Promise.resolve(true)
+  }
+}
+checker.registerService('test', service)
+```
+
+### `checkServiceByName`
+
+The `checkServiceByName` method allow check registered service by name.
+
+**Syntax**
+```js
+checkServiceByName (name)
+```
+
+**Parameters**
+1. `name`: (string) - The service name which you want check.
+
+**Example**
+```js
+const checker = new CheckerService()
+
+// We believe that you are already was
+// register service
+// else this will not work ¯\_(ツ)_/¯
+
+checker.checkServiceByName('test')
+```
+
+### `checkService`
+
+The `checkService` method allow check any service which have the `check` function.
+
+**Syntax**
+```js
+checkService (instance)
+```
+
+**Parameters**
+1. `instance`: - Any object, which have the `check` function.
+
+**Example**
+```js
+const checker = new CheckerService()
+
+const service = {
+  async check () {
+    return Promise.resolve(true)
+  }
+}
+
+checker.checkService(service)
+  .then(() => console.log('Service available'))
+
+// Service available
+```
+
+### `getAllServices`
+
+The `getAllServices` method return an array with `CheckerItem`.
+
+**Syntax**
+```js
+getAllServices ()
+```
+
+**Example**
+```js
+const checker = new CheckerService()
+
+// Register service
+
+const allServices = checker.getAllServices()
+```
+
+### `getServiceByName`
+
+The `getServiceByName` method return a `CheckerItem` by name if exists else `undefined`.
+
+**Parameters**
+1. `name`: (string) - The service name which you want get.
+
+**Syntax**
+```js
+getServiceByName (name)
+```
+
+**Example**
+```js
+const checker = new CheckerService()
+
+const originService = {
+  async check () {
+    return Promise.resolve(true)
+  }
+}
+
+checker.registerService('test', originService)
+
+const service = checker.getServiceByName('test')
+
+(originService === service.instance) // true
+```
+
+## Example usage
+
+Form more usage examples see `checker.spec.ts`
+
+```js
+const checker = new CheckerService()
+
+const workingService = {
+  async check () {
+    return Promise.resolve(true)
+  }
+}
+const notWorkingService = {
+  async check () {
+    return Promise.reject(new Error('Some reason of error'))
+  }
+}
+
+checker.registerService('workingService', workingService)
+checker.registerService('notWorkingService', notWorkingService)
+
+const allServices = checker.getAllServices()
+
+const resultsOfCheckServices = allServices.reduce(
+  async (previousPromise, service) => {
+    const acc = await previousPromise
+
+    const isServiceActive = await checker.checkService(service.instance)
+      // You can catch error as you need.
+      // In this example error was ignored and returned false
+      .catch(() => false)
+    acc.push(isServiceActive)
+
+    return Promise.resolve(acc)
+  },
+  Promise.resolve([])
+)
+
+resultsOfCheckServices
+  .then((checkResult) => {
+    consol.log(checkResult) // [true, false]
+  })
+```