Update custom algorithms example

[OR13]

This pull request updates the readme section describing custom algorithms, and provides an example test.

The current instructions in this section are not very helpful:

https://github.com/search?q=repo%3Averaison%2Fgo-cose%20RegisterAlgorithm&type=code

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.27%. Comparing base (92fbe95) to head (223b104).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #209      +/-   ##
==========================================
- Coverage   92.35%   91.27%   -1.09%     
==========================================
  Files          12       12              
  Lines        1622     2005     +383     
==========================================
+ Hits         1498     1830     +332     
- Misses         68      119      +51     
  Partials       56       56              

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