Add a spam example to the unacceptable list

[markmcd]

GitHub points users to the code of conduct when they are blocked, but we don't mention spamming explicitly in the list.

We get a fair bit of spammy comment activity on our repositories (e.g.), and I can update our own COC but this seems relevant to any Google project on GitHub so I am proposing the change here.

[emckean]

Thanks Mark! I like this addition but I think we should narrow it to "Sending spam or other unsolicited commercial messages" because "non-productive commentary" is too broad. I believe people recognize spam when they see it but would have reasonable disagreement about what's "non-productive" (e.g. sending memes, community in-jokes, etc.)