Add @help_ml and @help_git Slack groups

Author: emiliom

resources/logistics/getting_help.md

@@ -25,6 +25,8 @@ You are also welcome to direct message the helpers if you don't feel comfortable
 - `@help-projects` - For help with managing projects, for instance asking how to set up a new repo, or organize meetings.
 - `@help-python` - Questions about Python programming.
 - `@help-r` - Questions about R programming.
+- `@help-ml` - Questions about machine learning
+- `@help-git` - Questions about git
 - `@help-tutorials` - For help with tutorial logistics, like cloning the notebooks.
 - `@help-website` - If there are any issues with the website tag us to take a look.