Add README for PixBary validation

Alignment/OfflineValidation/README_PixBary.md

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+# Pixel Barycentre Validation
+Currently only the `single` type of job is implemented in the All-in-One Tool, but the execution of the plotting script `plotBaryCentre_VS_BeamSpot.py` could be added in the future as a `trend` job.
+
+## Extraction from TrackerAlignmentRcd - PixelBarycentreAnalyzer_cfg.py
+This config runs `PixelBaryCentreAnalyzer` on an empty source, after configuring the GlobalTag and additional conditions for a given alignment.
+The following parameters are expected/accepted in the json/yaml configuration file:
+```
+validations:
+    PixBary:
+        single:
+            <job_name>:
+                <options>
+```
+
+The following options are understood:
+
+Variable | Default value | Explanation/Options
+-------- | ------------- | --------------------
+firstRun | 290550 | The first run to process (inclusive)
+lastRun | 325175 | The last run to process (inclusive)
+lumisPerRun | 1 | The number of LumiSections tested for a change in the TrackerAlignmentRcd in each run
+alignments | None | List of alignments for which this validation is run