Git Harvest Plugin and Tutorial

[SKernchen]

Closes #255 .
This Feature shows a Tutorial for writing plugins for HERMES. To do that the git harvest plugin will be in another project.
Currently in https://github.com/hermes-hmc/hermes-git

[SKernchen]

My idea is to open a new public repository named plugins/ and add harvest-git there. Then I would like to use this plugin in the workflow source code, so HERMES would actively use it.

Update: you can't have group repos, so we decided for a name convention. Every plugins name should be hermes-plugin-{plugin name}

[led02]

Mostly formatting, but some minor (content) changes are also proposed.
There is one line that tells the wrong story...

[SKernchen]

We should maybe keep the name convention hermes-plugin-git through the whole project and not change to hermes_git but to hermes_plugin_git