Adds missing instructions to source installation

[Voldivh]

🦟 Bug fix

Fixes gazebosim/gazebo_test_cases#466

Summary

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[codecov]

Codecov Report

Merging #1322 (93f62c6) into sdf14 (881cbb7) will not change coverage.
The diff coverage is n/a.

❗ Current head 93f62c6 differs from pull request most recent head a5a5c57. Consider uploading reports for the commit a5a5c57 to get more accurate results

@@           Coverage Diff           @@
##            sdf14    #1322   +/-   ##
=======================================
  Coverage   87.47%   87.47%           
=======================================
  Files         134      134           
  Lines       17751    17751           
=======================================
  Hits        15528    15528           
  Misses       2223     2223