CONTRIBUTING: add API guidelines link and update the style guid… (#4785)
CONTRIBUTING: add API guidelines link and update the style guid… (#4785)

Signed-off-by: Lonng <8ed72e0d00108201f79dd2bb5270f106b3e5c19c@lonng.org>
c4pt0r/tikv,BusyJay/tikv,pingcap/tikv,BusyJay/tikv,BusyJay/tikv,BusyJay/tikv,pingcap/tikv