* Signed-off-by: luzizhuo <496521310@qq.com>

1. Update the way of member nomination to PR instead, because it's not so efficient at the community meeting.
2. Propose to add the nominee's contribution and the reason in the nomination PR
3. Add description of how the nomination is approved

* Update GOVERNANCE.md
Signed-off-by: luzizhuo <496521310@qq.com>
Co-authored-by: Calvin Weng <wenghao@pingcap.com>

* Update form
Signed-off-by: luzizhuo <496521310@qq.com>
Co-authored-by: Calvin Weng <wenghao@pingcap.com>
Co-authored-by: Ti Chi Robot <ti-community-prow-bot@tidb.io>