Add Document to Store Simple Tax Records

[GoFroggyRun]

This PR adds a new document for Tax-Calculator that stores some interesting (counter-intuitive) phenomenon we have encountered so far: odd (high) marginal tax rate, #555, and optimization puzzle, #752, for the moment. Contents included in the document are:

1. Introduction of this document
2. Different situations where each section contains:
   • Overview of the situation
   • Brief explanation of the reason
   • Simple tax record that would yield the situation
   • Guide on how to use Tax-Calculator to reproduce the situation

This document helps users and developers to have a good grasp of the reasons behind those "weird" phenomenons. This document will continue to be updated once interesting units/situations have been found.

Any comments, concerns or remarks on the name of this document, content of the document or format of the document would be mostly welcomed.

@martinholmer @MattHJensen @Amy-Xu @feenberg

[codecov-io]

Current coverage is 98.12% (diff: 100%)

Merging #764 into master will not change coverage

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[martinholmer]

@GoFroggyRun, I would suggest a different name for the new markdown file in pull request #764.

Instead of RECORD.md (which makes it sound as if you are going to discuss just one record), why not call the new file: ODD_RECORDS.md. This name indicates that more than one record is going to be discussed and that the records being discussed are "special" or "unusual" or "odd" in some way.

@MattHJensen @feenberg @Amy-Xu @zrisher

[GoFroggyRun]

@martinholmer, thanks for your comment regarding the name for the markdown.
ODD_RECORDS.md seems much more appropriate to me as well.

Please let me know if you have any other suggestions or comments.

[MattHJensen]

Merging. Thanks a lot @GoFroggyRun. Very much looking forward to more contributions on this documentation from you and others in the future. @GoFroggyRun, I think this resolves #753 -- if you agree, could you close the issue?