Update README.md - Link to SDF Spec

[ethanholter]

Simple link to the SDF specification. This is in relation to issue 103. This isn't the entire scope of what is asked in the original issue, however a link like this would've saved me an immense amount of time so I think this is much better than nothing.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.07%. Comparing base (624e503) to head (3d6284d).
Report is 14 commits behind head on gz-sensors9.

Additional details and impacted files

@@               Coverage Diff               @@
##           gz-sensors9     #505      +/-   ##
===============================================
- Coverage        72.47%   72.07%   -0.41%     
===============================================
  Files               37       37              
  Lines             5108     5174      +66     
===============================================
+ Hits              3702     3729      +27     
- Misses            1406     1445      +39     

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