Remove npm install from README

[Jhappy77]

The readme recommends using npm install after cloning the repository, and adds that you can use yarn install as an alternative. I think that it should only recommend using yarn install.

Reasons:

• The repo comes with a premade yarnlock and no package-lock
• The CI checks use yarn. If a user uses npm instead of yarn to install their packages, and continues to use npm, these checks will fail because yarn will realize the yarnlock is outdated.

I think it would be best to strongly encourage people to use yarn with this setup since that's what everything is written with. This will avoid confusing newcomers - and if converting to npm easily remains important, perhaps a conversion process could be described in the README.

[jpedroschmitz]

Hey, @Jhappy77, that makes sense! I'll open a PR to remove it!

Thanks a lot =D