feat(monorepo): Add contributing guide

[michelguerin]

Details the installation steps to contribute as a developer.

• Link from readme to contributing guide + refacto
• Add .env.example file for needed environment variables
• Add .env to .gitignore

[trusz]

We already have a CONTRIBUTING.md in the core package: https://github.com/openscd/open-scd/blob/main/packages/core/CONTRIBUTING.md

Have you took that into consideration?

[michelguerin]

We already have a CONTRIBUTING.md in the core package: https://github.com/openscd/open-scd/blob/main/packages/core/CONTRIBUTING.md

Have you took that into consideration?

Oups, didn't see this. I'll merge these to file in favor of the root one. I'll make a new proposition in the next commit.

[michelguerin]

@trusz Update done : I removed the contributing guide from the core package and merge its content to root.