Rename the plugin and and update readme

[DHaussermann]

Maybe this is far more obvious for the intended obvious but to people not familiar with Prometheus AlertManager it may not be obvious what this plugin does.
I thought this plugin maybe communicated with Prometheus directly or maybe even with Graphana somehow (which also has an add-on called AlertManager). From what I see, the read-me here https://github.com/cpanato/mattermost-plugin-alertmanager#readme does not explicitly say that it communicates Prometheus AlertManager it's only implied by the screen shots.

Therefore I would suggest we rename the plugin to prometheus-alertmanager
and Update the top level header in the screenshot here.

[DHaussermann]

I don't have permissions to label things in tis repo but this in an enhancement request.

[cpanato]

are you willing to update the readme for that?