Add README for timing minitest.

Signed-off-by: Keith Rothman <537074+litghost@users.noreply.github.com>

minitests/timing/README.md

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+Timing minitest
+===============
+
+This minitest uses Vivado to compile a design and extracts the relevant timing
+metadata from the design, e.g. what are the nets and how was the design routed.
+
+For each clock path, the final timing is provided for each of the 4 corners of
+analysis.
+
+From the timing metadata, ``create_timing_worksheet_db.py`` creates a worksheet
+breaking down the interconnect timing calculation and generating a final
+comparision between the reduced model implemented in prjxray and the Vivado
+timing results.
+
+Model quality
+=============
+
+The prjxray timing handles most nets +/- 1.5% delay.  The large exception to
+this is clock nets, which appear to use a table lookup that is not understood
+at this time.
+
+Running the model
+=================
+
+The provided Makefile will by default compile all examples.  It a specific design
+family is desired, the family name can be provided.  If a specific design within
+a family is desired, use ``<family name>_<iter>``.
+
+Example:
+```
+# Build all variants of the DFF loopback test
+make dff
+# Build only DESIGN_NAME=dff ITER=63
+make dff_63
+```