Update CONTRIBUTING.md (#320)

Mention markdown style requirement and rearrange sections.

CONTRIBUTING.md

@@ -6,6 +6,11 @@ Before you start - see OpenTelemetry general
 [contributing](https://github.com/open-telemetry/community/blob/master/CONTRIBUTING.md)
 requirements and recommendations.
 
+## Sign the CLA
+
+Before you can contribute, you will need to sign the [Contributor License
+Agreement](https://identity.linuxfoundation.org/projects/cncf).
+
 ## Proposing a change
 
 Significant changes should go through the [RFC process](https://github.com/open-telemetry/rfcs).
@@ -15,11 +20,35 @@ Significant changes should go through the [RFC process](https://github.com/open-
 Specification is written in markdown format. Please make sure files are rendered
 OK on GitHub.
 
+### Markdown style
+
+Markdown files should be properly formatted before a pull request is sent out.
+In this repository we follow the
+[markdownlint rules](https://github.com/DavidAnson/markdownlint#rules--aliases)
+with some customizations. See [mdlstyle](.mdlstyle.rb) or
+[settings](.vscode/settings.json) for details.
+
 We highly encourage to use line breaks in markdown files at `80` characters
 wide. There are tools that can do it for you effectively. Please submit proposal
 to include your editor settings required to enable this behavior so the out of
 the box settings for this repository will be consistent.
 
+To check for style violations, use
+
+```bash
+# Ruby and gem are required for mdl
+gem install mdl
+mdl -c .mdlrc .
+```
+
+To fix style violations, follow the
+[instruction](https://github.com/DavidAnson/markdownlint#optionsresultversion)
+with the Node version of markdownlint. If you are using Visual Studio Code,
+you can also use the `fixAll` command of the
+[vscode markdownlint extension](hhttps://github.com/DavidAnson/vscode-markdownlint).
+
+### Misspell check
+
 In addition, please make sure to clean up typos before you submit the change.
 
 To check for typos, use
@@ -34,9 +63,4 @@ To quickly fix typos, use
 
 ```bash
 make misspell-correction
-```
-
-## Sign the CLA
-
-Before you can contribute, you will need to sign the [Contributor License
-Agreement](https://identity.linuxfoundation.org/projects/cncf).
+```