Add developer guide

[taufeeque9]

Description

Add a basic developer guide to describe the implementation of the library.

[codecov[bot]]

Codecov Report

Merging #525 (a41946a) into master (32467bf) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #525   +/-   ##
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  Coverage   97.28%   97.28%           
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  Files          88       88           
  Lines        8030     8030           
=======================================
  Hits         7812     7812           
  Misses        218      218           

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[tomtseng]

@tomtseng what did you find confusing when you started working on imitation?

I agree that the code in scripts/ was difficult to follow when I was debugging a test in test_scripts.py. But I also didn't read the docs for Sacred very much — it might be enough to just link to the relevant Sacred doc page(s)