replace npm with yarn in CONTRIBUTING.md about ak-phrase.js

[arshadkazmi42]

actually it supports both, I was using yarn while building it and while setting up the repository, I think I used npm everywhere. We can centralize it to use either npm or yarn and accordingly update yarn.lock or package-lock.json file

[cdrani]

I know it supports both, but the consensus, I believe, is to develop using only one.

We highly recommend you to delete the package-lock.json file if you decide to use yarn in order to avoid future confusion and possible consistency issues.

https://yarnpkg.com/blog/2018/06/04/yarn-import-package-lock/

[arshadkazmi42]

True. We should not use npm and yarn both, it can make the maintenance of the library difficult

[cdrani]

PR incoming.