Update contributing file

[vinaypuppal]

Under https://github.com/coderplex/coderplex/blob/master/CONTRIBUTING.md#running-this-project-locally add this as a note

So our team's official policy (for now) is:
We only use Yarn as our official Node package manager, and so we request you to use Yarn instead of npm and commit yarn.lock file.

[vinaypuppal]

Added in #86