[REVIEW] README cleanup

[randerzander]

closes #838

User feedback about cuDF's README indicated it was too dense, with too much information. Users preferred to see a simple motivating example, followed by install instructions, with links to any lengthy build instructions.

Changes:

1. Simplify project description, removing filler wording where possible
2. Add cudf/python example
3. Add links to API docs and notebooks
4. Move build instructions to BUILD.md
5. Fix inconsistency between rapids.ai specified conda/pip install arguments

[mike-wendt]

@randerzander @kkraus14 Instead of creating a new BUILD.md file, can we add this to the end of CONTRIBUTING.md like PyTorch does? https://github.com/pytorch/pytorch/blob/master/CONTRIBUTING.md

[mike-wendt]

@randerzander @kkraus14 Instead of creating a new BUILD.md file, can we add this to the end of CONTRIBUTING.md like PyTorch does? https://github.com/pytorch/pytorch/blob/master/CONTRIBUTING.md

That had been the plan for the 0.5 release but we weren't ready and deferred.

[randerzander]

@mike-wendt thanks for the excellent suggestion. I made those changes.

I'll open new PRs to cuML, cuGraph, and cuStrings to follow the same pattern.