Improvements to contributing docs

Merge Request Checklist

• Link to an issue
• Include the proposed fix or feature
• Include and update tests for the modified code
• Include a documentation change
• Add a CHANGELOG.md entry in the Unreleased section

Once your merge request is complete, please assign it a Meltano maintainer for review cycle. Once the review cycle finished, the reviewer shall approve the merge request so it can be merged by any Meltano maintainer.

Problem

Solution

This MR:

• update instruction to specify creating venv specific for meltano-development. This is better because creating a venv called venv is what every tutorial recommends and can lead to conflicts when installing. By naming it meltano-development, you avoid a conflict with the meltano venv created in the installation instructions.
• add comments about yarn error