Add note about k-D tree implementation

[btjanaka]

Description

Small note to memorialize our choice of scipy's k-D tree over sklearn's k-D tree.

TODO

Questions

Status

• [x] I have read the guidelines in CONTRIBUTING.md
• [x] I have formatted my code using yapf
• [x] I have tested my code by running pytest
• [x] I have linted my code with pylint
• [ ] ~I have added a one-line description of my change to the changelog in HISTORY.md~
• [x] This PR is ready to go