Update discrete systematic README (Ultrasurface use)

[LeanderFischer]

The previous version described the use of scripts that aren't part of pisa. This change therefore:

• removes the pisa unrelated descriptions
• updates the use instructions with specific config and pipeline example

[thehrh]

• general suggestion: keep documentation focussed on the stage itself to reduce future maintenance cost (import/include syntax and pipeline config examples can already be found elsewhere)
• other than that, just a few minor (non-critical) remarks

[LeanderFischer]

• general suggestion: keep documentation focussed on the stage itself to reduce future maintenance cost (import/include syntax and pipeline config examples can already be found elsewhere)

Yes, that's a good point, I removed the import part, but I do think it's valuable to point out at which position this stage should be included in the pipeline order, since it's different to the hypersurface stage.

[thehrh]

Looks good to me, thank you. Merging now.