Add overview of different alignment routes to README

[apeltzer]

Description of feature

As per suggestion of @Zethson , it would be great to add a Mermaid graph to the readme to help users out who might want to do analysis of various types with the pipeline, e.g. scATAC + scRNA multi-ome analysis.

I might add one that has the scrnaseq domain covered, e.g. for alevin-fry, cellranger, kallisto+bustools, STARsolo.

We can then extend this in PRs for Universc @TomKellyGenetics and the multi-ome analysis that @heylf brought up if the community deems this useful.

Another discussion we might have to have is a rename of the pipeline, as it can do way more than just scrnaseq in such cases - good naming is however tricky here. Also linking to discussion on Slack https://nfcore.slack.com/archives/CE4K7FEHE/p1667987775811819

[grst]

This is not rendered on the website:

Probably easiest to produce the graph elsewhere and upload a png.