updated build instructions

[malleshamdasari]

 Some platforms, the linker may be stripping out the self-registering
 classes FileReaderFactory uses to read inputs. You can use the following
 ways to avoid this problem.

 target_link_libraries(native-lib -Wl,--whole-archive draco -Wl,--no-whole-archive)

 or

 target_link_libraries(native-lib -Wl,-force_load draco)

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