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

Author: JFreegman

INSTALL.md

@@ -147,6 +147,7 @@ There are some options that are available to configure the build.
 | `MSVC_STATIC_SODIUM`   | Whether to link libsodium statically for MSVC.                                                | ON or OFF                                                                 | OFF                                               |
 | `MUST_BUILD_TOXAV`     | Fail the build if toxav cannot be built.                                                      | ON or OFF                                                                 | OFF                                               |
 | `NON_HERMETIC_TESTS`   | Whether to build and run tests that depend on an internet connection.                         | ON or OFF                                                                 | OFF                                               |
+| `EXPERIMENTAL_API`     | Install the experimental private API header.                                                  | ON or OFF                                                                 | OFF                                               |
 | `STRICT_ABI`           | Enforce strict ABI export in dynamic libraries.                                               | ON or OFF                                                                 | OFF                                               |
 | `TEST_TIMEOUT_SECONDS` | Limit runtime of each test to the number of seconds specified.                                | Positive number or nothing (empty string).                                | Empty string.                                     |
 | `USE_IPV6`             | Use IPv6 in tests.                                                                            | ON or OFF                                                                 | ON                                                |