implementation-notes.md

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Write the implementation overview in
https://github.com/phetsims/natural-selection/blob/master/doc/implementation-notes.md.

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Done, closing.

I tried something new in these notes. For model and view, I provided bullet lists of things that you might want to find out more about, and pointers to the relevant code. For example:

Here are pointers to some of the major features of the model:

• Main animation loop: NaturalSelectionModel.step
• Bunny motion: Bunny.move
• Bunny appearance: Phenotype, GenePair.getVisibleAllele
  ...

We'll see if devs find this useful during code review.

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Reopening for @jonathanolson's feedback in #203 (code review).

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Over in #203 (comment), @jonathanolson said:

Does implementation-notes.md adequately describe the implementation, with an overview that will be useful to future maintainers? Please provide feedback in #203.

Incredibly useful and perfect! Didn't see any changes to recommend. Thanks!

I meant to say "provide feedback in #151", not #203. But I'm glad it was useful, and I'm going to close this issue.