Docs: Add contrast ration check notice into themes contributing guide

CONTRIBUTING.md

@@ -55,6 +55,9 @@ All you need to do is edit the [themes/index.js](./themes/index.js) file and add
 > **Note**
 > Keep in mind that we already have a vast collection of different themes. To keep their number manageable, we began to add only themes supported by the community. Your pull request with theme addition will be merged once we get enough positive feedback from the community in the form of thumbs up (see [#1935](https://github.com/anuraghazra/github-readme-stats/issues/1935#top-themes-prs)). Remember that you can also support themes of other contributors that you liked to speed up their merge.
 
+> **Note**
+> Before submitting pull request, please make sure that your theme pass WCAG 2.0 level AA constrast ration test. You can use [this tool](https://webaim.org/resources/contrastchecker/) to check it.
+
 ## Any contributions you make will be under the MIT Software License
 
 In short, when you submit changes, your submissions are understood to be under the same [MIT License](http://choosealicense.com/licenses/mit/) that covers the project. Feel free to contact the maintainers if that's a concern.