Improving CONTRIBUTING.md

The CONTRIBUTING.md is outdated and needs some updates, for example it’s not mentioned anything about building or running the project.

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For building the first time:

Unix

yarn && npx lerna bootstrap && npx lerna run build

Windows

yarn
npx lerna bootstrap
npx lerna run build

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For building local packages it’s possible to run only

yarn build

in every folder with a package.json file

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For running demos

yarn start

or

yarn serve

where the start commands returns an error (this could be another improvement)

[utkarshhbtu]

can i work on this issue

[tamoghna-dey]

I just submitted a PR.