message: docs(write plugin): enhance doc
diff: BScroll.use(MyPlugin)
  const bs = new BScroll('.wrapper', {
  - myPluginOptions: {
  + myPlugin: {
  scrollText: 'I am scrolling',
  scrollEndText: 'Scroll has ended'
  },
  // or
  - myPluginOptions: true
  + myPlugin: true
  })
  // Use the event that is proxied to bs by plugin
  ```typescript
  import BScroll from '@better-scroll/core'
  -export type myPluginOptions = Partial<myPluginConfig> | true
  +export type MyPluginOptions = Partial<MyPluginConfig> | true
  -type myPluginConfig = {
  +type MyPluginConfig = {
  scrollText: string,
  scrollEndText: string
  }
  @@ -46,11 +46,11 @@ interface PluginAPI {
  declare module '@better-scroll/core' {
  interface CustomOptions {
  - myPluginOptions?: myPluginOptions
  + myPlugin?: myPluginOptions
  }
  interface CustomAPI {
  - myPluginOptions: PluginAPI
  + myPlugin: PluginAPI
  }
  }
  ```
  @@ -72,8 +72,8 @@ The advantage of this is that when the `myPlugin` plugin is imported and BetterS
  ```typescript
  export default class MyPlugin implements PluginAPI {
  - static pluginName = 'myPluginOptions'
  - public options: myPluginConfig
  + static pluginName = 'myPlugin'
  + public options: MyPluginConfig
  constructor(public scroll: BScroll){
  this.handleOptions()
  @@ -92,10 +92,10 @@ The advantage of this is that when the `myPlugin` plugin is imported and BetterS
  import { extend } from '@better-scroll/shared-utils'
  export default class MyPlugin {
  private handleOptions() {
  - const userOptions = (this.scroll.options.myPluginOptions === true
  + const userOptions = (this.scroll.options.myPlugin === true
  ? {}
  - : this.scroll.options.myPluginOptions) as Partial<myPluginConfig>
  - const defaultOptions: myPluginConfig = {
  + : this.scroll.options.myPlugin) as Partial<MyPluginConfig>
  + const defaultOptions: MyPluginConfig = {
  scrollText: 'I am scrolling',
  scrollEndText: 'Scroll has ended'
  }
  