[DOC] Adding development docs to make sure it is clear how to do dev

[adam2392]

Currently, it is unclear sometimes how to setup, or carry out development locally. This document seeks to address this.

Please let me know if this addresses any confusion you have had, or have.

Here is the file rendered for your ease-of-review: https://github.com/neurodata/scikit-tree/blob/e1aafe47cb266e6fa74cab5ef6093a2cd4d60942/DEVELOPING.md

Before submitting

• I've read and followed all steps in the Making a pull request
  section of the CONTRIBUTING docs.
• I've updated or added any relevant docstrings following the syntax described in the
  Writing docstrings section of the CONTRIBUTING docs.
• If this PR fixes a bug, I've added a test that will fail without my fix.
• If this PR adds a new feature, I've added tests that sufficiently cover my new functionality.

After submitting

• All GitHub Actions jobs for my pull request have passed.

[sampan501]

Few minor comments, but otherwise we should be good to merge

[PSSF23]

For some reason the commands 2 & 3 in the last section are not rendered as code, but the new doc looks good.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (983b846) 89.73% compared to head (b5c3703) 89.73%.
Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   89.73%   89.73%           
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  Files          41       41           
  Lines        3352     3352           
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  Hits         3008     3008           
  Misses        344      344           

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