Warn against deleting .julia folder with deved packages.

[baggepinnen]

The previous advice is potentially dangerous, this PR adds a warning for users who have called pkg> develop on any package

[fredrikekre]

I would not even mentioning it as an option but instead recommend something more targeted. .julia/packages/IJulia probably, and perhaps .julia/conda?

[stevengj]

Yes, I would just remove that advice entirely, and replace it with just removing .julia/conda.

Nowadays the package system is more robust than in the early days of IJulia, and it's pretty rare that you would need to remove .julia entirely.

[ViralBShah]

Update based on @stevengj's suggestion and merge?