update .editorconfig for Markdown

- add trim_trailing_whitespace to .editorconfig, because trailing spaces
  in Markdown have the function of <br>
- also added some content in the CONTRIBUTING doc about Editorconfig

.editorconfig

@@ -10,4 +10,5 @@ trim_trailing_whitespace = true
 insert_final_newline = true
 
 [*.md]
+trim_trailing_whitespace = false
 insert_final_newline = false

CONTRIBUTING.md

@@ -73,6 +73,7 @@ Here are some hints for getting eslint setup in your favorite editor:
 | IntelliJ | Settings » Languages & Frameworks » JavaScript » Code Quality Tools » ESLint |
 | vi | [scrooloose/syntastic](https://github.com/scrooloose/syntastic) |
 
+Another tool we use for enforcing consistent coding style is Editorconfig, which can be set up by installing a plugin in your editor that dynamically updates its configuration. Take a look at the [Editorconfig](http://editorconfig.org/#download) site to find a plugin for your editor, and browse our [`.editorconfig`](https://github.com/elastic/kibana/blob/master/.editorconfig) file to see what config rules we set up.
 
 ### Testing and building