Kneed

almost installable through `conda-forge`.

update setup.py to render README.md in pypi.

Incorporate PRs from:

kev494 use `warnings` instead of `print` 4e898c072ce0c6ce7da6f14151a05fb6e0130bc8
big-o add `elbow` property to `KneeLocator` f2666b8fa58250b972f2281b707ccba597e2e65b

I also updated the docs a bit.

Finds the knee when there is a negative threshold.
Also added some additional tests.

Implements tests via CI and detects knee point more accurately and explicitly.