Add instructions to build and run examples

[chapulina]

New feature

Summary

Users may not immediately know how to use the examples. I didn't see any instructions on how to build and run them, so I added a very short README.

Test it

Try following the instructions on the README to see if they work.

Checklist

• Signed all commits for DCO
• Added tests
• Added example world and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review
  another open pull request
  to support the maintainers

Note to maintainers: Remember to use Squash-Merge

[codecov]

Codecov Report

Merging #192 (9d486f9) into ign-math6 (5a4321b) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           ign-math6     #192   +/-   ##
==========================================
  Coverage      99.20%   99.20%           
==========================================
  Files             63       63           
  Lines           6020     6020           
==========================================
  Hits            5972     5972           
  Misses            48       48           

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