Document how to use sarek without an igenomes genome

[FriederikeHanssen]

Document how to use sarek without an igenomes genome in different stages:

• no BWA Indices
• no known indels
• snpEff/VEP on another organism.

Lots of people have problems figuring out how to do this.
We should come up with a better of explaining it.

[gorgitko]

Hello, I would also suggest to document the used iGenomes files, i.e. what is their origin, by which tools in the pipeline are they used / what is their purpose, etc.

Thanks! ❤️

[maxulysse]

Duplicate of #291

But nowaways, basically, we need to do:
--genome null --igenomes_ignore and specify all necessary files.

[FriederikeHanssen]

@gorgitko I added a table for the origin of the reference genomes now in #673