docs(faraday-ui): add documentation for faraday helpers

packages/component-faraday-ui/src/helpers/helpers.md

-## Hindawi utility HOCs.
+## Hindawi Helpers
+
+_Utility HOCs_
 
 * [withCountries](#withCountries)
 * [withFetching](#withFetching)
@@ -8,6 +10,17 @@
 * [withRoles](#withRoles)
 * [withZipDownload](#withZipDownload)
 
+_HOCs used for files drag and drop_
+
+* [withFileSectionDrop](#withFileSectionDrop)
+* [withNativeFileDrop](#withNativeFileDrop)
+
+_Utility functions_
+
+* [handleError](#handleError)
+
+# Utility HOCs
+
 ## withCountries
 
 Injects `countries` and `countryLabel` as props.
@@ -28,6 +41,8 @@ const Wrapped = ({ countries, countryLabel }) => (
     <span>Selected country: {countryLabel('RO')}</span>
   </div>
 )
+
+export default withCountries(Wrapped)
 ```
 
 ## withFetching
@@ -166,3 +181,98 @@ This HOCs assumes the following props are present on the wrapped component:
 ### withZipDownload props
 
 * `downloadFiles: () => any`: download all the fragment's file as a zip
+
+# Files drag and drop
+
+## withFileSectionDrop
+
+HOC used to provide drop functionality to the `FileSection` component. It's main purpose is to change a file from one list to another. This is usually done in a callback called `changeList` that should be provided to the wrapped component.
+
+This HOC assumes the wrapped component has the following props:
+
+* `files: [{id: string, ...}]`: the list of files passed to the wrapped component
+* `setError: (error: string) => any`: error setting callback
+* `listId: string`: the current list id
+* `allowedFileExtensions: [string]`: the allowed files
+* `maxFiles: number`: the maximum number of files allowed
+* `changeList: (fromListId: string, toListId: string: fileId: string)`: callback called if all the conditions are met (allowed files, number of files, etc)
+
+```js
+import { compose, withHandler, withProps } from 'recompose'
+import { FileSection, withFileSectionDrop } from 'pubsweet-component-faraday-ui'
+
+const Wrapped = compose(
+  withProps({
+    files: [...],
+    listId: 'CoverLetter',
+    maxFiles: 3,
+    allowedFileExtensions: ['pdf'],
+  }),
+  withHandlers({
+    changeList: () => (fromListId, toListId, fileId) => {
+      // do the actual change here
+    }
+  }),
+  withFileSectionDrop,
+)(FileSection)
+
+export default Wrapped
+```
+
+## withNativeFileDrop
+
+HOC used to provide native file drop functionality to the `FileSection` component. It's purpose is to do something when dragging files from the computer's hard drive into the app. _This HOC allows only single items! Dragging multiple items into the wrapped component will only handle the first item!_
+
+This HOC assumes the wrapped component has the following props:
+
+* `files: [{id: string, ...}]`: the list of files passed to the wrapped component
+* `setError: (error: string) => any`: error setting callback
+* `allowedFileExtensions: [string]`: the allowed files
+* `maxFiles: number`: the maximum number of files allowed
+* `onFileDrop: (file: File)`: callback called when a valid file is dropped
+
+```js
+import { compose, withHandler, withProps } from 'recompose'
+import { FileSection, withNativeFileDrop } from 'pubsweet-component-faraday-ui'
+
+const Wrapped = compose(
+  withProps({
+    files: [...],
+    listId: 'CoverLetter',
+    maxFiles: 3,
+    allowedFileExtensions: ['pdf'],
+  }),
+  withHandlers({
+    onFileDrop: () => file => {
+      // do something with the dropped file
+    }
+  }),
+  withNativeFileDrop,
+)(FileSection)
+
+export default Wrapped
+```
+
+# Utility functions
+
+## handleError
+
+Function that parses the server error. Calls the passed function with the parsed error.
+
+Has the following signature:
+`(callbackFn: (parsedError) => any) => (e: Error) => any`
+
+```js
+const customErrorLogger = parsedError =>
+  console.error(`This is very handled: ${parsedError}`)
+
+// point free notation
+anAsyncOperation().catch(handleError(customErrorLogger))
+
+// can be used besides other function calls
+
+anAsyncOperation().catch(err => {
+  setFetching(false)
+  handleError(customErrorLogger)(err)
+})
+```