updated docs for commandline tool

This version should be an acceptable first shot for the commandline tool doc.

Some findings, I´d like to share:
* There should be a cross reference to the API docs of FSharp.Formatting and the cli tool, but the API reference [does not exist](http://tpetricek.github.io/FSharp.Formatting/metadata.html#) per today.
* The commandline tool is outdated with respect to `FSharp.MetadataFormat.Generate`, as it does not implement the options `publicOnly`and `otherFlags`. Most likely easy to update this, `FAKE` should not be harmed by it, anyway.
* It seems to be a wrong decision to use the [Gsscoder library](https://github.com/gsscoder/commandline) for cli argument parsing as there exists `Microsoft.FSharp.Compiler.SourceCodeServices.InteractiveChecker` (I stepped over it [here](https://github.com/fbmnds/FSharp.Formatting/blob/master/src/FSharp.MetadataFormat/Main.fs#L679)).