Update README in header checker for option ignored_linker_set_keys

NDK and platform Libraries are required to ensure the compatibility
across SDK version. However, some changes of a library that break its
ABI compatibility are necessary. Thus a ignored_linker_set_keys option
is added to let developers to skip some symobls.

Test: no need
Bug: 243903630
Change-Id: If6ca665189fdeec7c3e735a0f691dffd180ddc63
1 file changed