Made CONTRIBUTING guide more (windows-)user friendly

[aGitForEveryone]

closes #2889

I had quite a few issues when trying to follow the contributing guide on my Windows system (also because I have very limited experience with working with the JavaScript ecosystem). After connecting to @ann-marie-ward and @BSd3v to get my system working, I wanted to share the gained knowledge to future (beginning) Dash contributors.

I added extra instructions and explanation to make the process a bit friendlier to new developers (and working on Windows systems like I do). Some other explanations where made a bit more verbose.

Contributor Checklist

• [x] No tests were run as this is only a change to CONTRIBUTING.md

optionals

• [ ] I have added entry in the CHANGELOG.md (is this necessary?)