(doc) add a hint on using - vs. / for cli switches

improve user experience. thx for @asbjornu for guidance

Author: pniederlag

docs/usage/command-line.md

@@ -2,6 +2,8 @@
 
 If you want a command line version installed on your machine then you can use [Chocolatey](http://chocolatey.org) or [Homebrew](https://brew.sh/) to install GitVersion.
 
+> **Hint**: While documentation and help use '/' as command prefix the hyphen '-' is supported as well and is a better alternative for usage on \*nix systems. Example: *-output json vs. /output json*
+
 ## Chocolatey
 
 Available on [Chocolatey](http://chocolatey.org) under [GitVersion.Portable](http://chocolatey.org/packages/GitVersion.Portable).