Update docs.

Author: trekhleb

CONTRIBUTING.md

@@ -1,4 +1,5 @@
 ## Contributing
 
-As much as possible, try to follow the existing format of markdown and code.
-Don't forget to run `npm run lint` and `npm test` before submitting pull requests.
+- As much as possible, try to follow the existing format of markdown and code.
+- Don't forget to run `npm run lint` and `npm test` before submitting pull requests.
+- Make sure that **100%** of your code is covered by tests.