Document WeChat notification support

[juliusv]

WeChat support got added in prometheus/alertmanager#1059, but it's not documented yet on https://prometheus.io/docs/alerting/configuration/

[infoverload]

Can I take this one?

[juliusv]

@infoverload Sure!

[juliusv]

@infoverload Actually it seems this was already fixed in https://github.com/prometheus/docs/pull/977/files, but this issue didn't get autoclosed, maybe because the actual commits in that PR didn't contain a "Fixes: ..." notice?

[juliusv]

Oh. Because it hasn't been merged yet.