add README for fu directory

diff --git a/src/soc/fu/README.md b/src/soc/fu/README.md
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+# Pipelines
+
+In this directory are the pipelines.  The structure of each pipeline is
+as follows:
+
+* pipe_data.py: contains pipeline input and output data structures
+* XXXX_stage.py: files with function-specific stages
+* XXX_input_record.py: a PowerISA decoded instruction subset for this pipeline
+* pipeline.py: the actual pipeline chain, which brings all stages together
+
+# Common files
+
+* regspec.py: the register specification API.  used by each pipe_data.py
+* pipe_data.py: base class for pipeline pipe_data.py data structures
+* common_input_stage.py: functionality common to input stages (RA, RB)
+* common_output_stage.py: functionality common to output stages (SO, CA/32 etc.)