Improve "Running demo app locally"

[geekayush]

Summary

Running the demo app locally suggests using two package managers (yarn and npm) at the same time. However, it is advised to use a single package manager in a project. This might raise a problem as mentioned in #97. npm should be replaced with yarn to maintain the consistency in using package manager.

Expected Behavior

Running demo app locally should not crash into errors.

Current Behavior

Running demo app locally might crash into errors.

Steps to Reproduce

Follow the mentioned steps.

Possible Solution

Replace npm with yarn in order to have a single package manager in the demo app.