Update readme with intended design

README.md

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-# xia3
+# xia3
+
+xia3 is an evolution on the xia2 automated processing pipeline and aims to be a long-term replacement. Target science case is ~monochromatic diffraction by the rotation method (Xrays/electrons).
+
+It aims to implement the following main features:
+
+- processing of multi-crystal datasets (images to merged data) as standard. Processing the 'cows-pigs-people' example dataset should cover ~80% of the required features i.e. integration of sweeps (as an ensemble) and multiplex-style data reduction. Single crystal processing is logically a subset of this.
+- program agnostic processing, supporting DIALS and XDS processing. xia3 is mainly a small logic wrapper, running programs from available packages.
+- suitable parallelism/asynchronous processing e.g. ability to distribute integration jobs across multiple processors (on slurm + ?)
+- takes a 'working-directory' approach, i.e. if rerun processing with same parameters, only reprocess new data, to support updating processing during collection.
+- takes the good bits from xia2, e.g. logging, html reports
+- be as lightweight as possible to increase ease of maintainability and longevity. Python based.