README.md: Change contribution method (Only use github)
README.md: Change contribution method (Only use github)

review.gerrithub.io is not properly working.

Signed-off-by: Changhyeok Bae <2c9096dac729cabe46106702f373e29af6635d98@gmail.com>

meta-gir/meta-gir,meta-gir/meta-gir