Fixing documentation, adding explanatory comments to unit tests

[esimpsons3ti]

The documentation previously stated that an understanding of XPath was necessary to use the tool effectively. This has been replaced with a passage that instead demonstrates how to read and understand the XPath-based headers the tool creates.

The blackbox unit tests have been given explanatory block comments for each entry. Each block comment contains the equivalent command-line query and the golden copy file to compare the results against.

Fixes #19

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 71.20%. Comparing base (6f7e07c) to head (99acafa).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #30 +/- ## ======================================= Coverage 71.20% 71.20% ======================================= Files 1 1 Lines 587 587 Branches 134 134 ======================================= Hits 418 418 Misses 144 144 Partials 25 25 ```

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