docs: add the experimental api build option to INSTALL.md

[JFreegman]

This change is 

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e1fa5ca) 73.02% compared to head (12bc042) 72.84%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2708 +/- ## ========================================== - Coverage 73.02% 72.84% -0.19% ========================================== Files 149 149 Lines 30517 30517 ========================================== - Hits 22286 22229 -57 - Misses 8231 8288 +57 ```

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