Add SLACK_WEBHOOK secret to nodejs/node-v8

[mmarchini]

Slack webhook in the OpenJS Foundation slack (channel #nodejs-v8-canary) for important notifications on the nodejs/node-v8 repository.

Ref: nodejs/node-v8#171 (comment)

[mmarchini]

cc @nodejs/tsc @nodejs/community-committee

[mhdawson]

I should probably know the answer but @mmarchini does this mean that additional people who have access to nodejs/node-v8 will now have access to the secret?

[mmarchini]

No, the secret is only visible while I'm setting, after that GitHub will mask it on logs (and when editing users can only enter new values, the old value won't show up).

[mmarchini]

Cc @nodejs/community-committee need one more +1

[Trott]

+1 (but I'm not on CommComm....)

[bnb]

+1

[mmarchini]

Done