[regression] Add README explaining the test

tests/regression/README.md

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+# Regression tests
+
+The regression tests run zstd in many scenarios and ensures that the size of the compressed results doesn't change. This helps us ensure that we don't accidentally regress zstd's compression ratio.
+
+These tests get run every night by CircleCI. If the job fails you can read the diff printed by the job to ensure the change isn't a regression. If all is well you can download the `results.csv` artifact and commit the new results. Or you can rebuild it yourself following the instructions below.
+
+## Rebuilding results.csv
+
+From the root of the zstd repo run:
+
+```
+# Build the zstd binary
+make clean
+make -j zstd
+
+# Build the regression test binary
+cd tests/regression
+make clean
+make -j test
+
+# Run the regression test
+./test --cache data-cache --zstd ../../zstd --output results.csv
+
+# Check results.csv to ensure the new results are okay
+git diff
+
+# Then submit the PR
+```