Update INSTALL.md

Add note for build option `-DOPENTELEMETRY_INSTALL`.

INSTALL.md

@@ -85,6 +85,7 @@ You can link OpenTelemetry C++ SDK with libraries provided in
    - `-DWITH_OTLP_GRPC=ON` : To enable building OTLP GRPC exporter.
    - `-DWITH_OTLP_HTTP=ON` : To enable building OTLP HTTP exporter.
    - `-DWITH_PROMETHEUS=ON` : To enable building prometheus exporter.
+   - [`-DOPENTELEMETRY_INSTALL=ON`](https://github.com/open-telemetry/opentelemetry-cpp/blob/main/sdk/CMakeLists.txt): To install `libopentelemetry_common.so` needed for external code linking that rely on `ld`.
 
 3. Once the build configuration is created, build the CMake targets - this
    includes building SDKs and unittests for API and SDK. Note that since API is