Add header to readme link table

[tpoliaw]

Having the source and repo link as a 'normal' row make it clearer what
is going on and stops 'Source' looking like a description of the other
link types.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.11%. Comparing base (664403b) to head (82e2228).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1848   +/-   ##
=======================================
  Coverage   99.11%   99.11%           
=======================================
  Files         318      318           
  Lines       12331    12331           
=======================================
  Hits        12222    12222           
  Misses        109      109           

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[Villtord]

Awesome!