added updated readme

etc/peer/README.md

+# PEER Test Cases
+
+The PEER PSHA verification project developed a number of source models for the purpose of
+examining the sensitivity of seismic hazard to different PSHA implementations (codes).
+These test cases are used as end-to-end unit tests in `nshmp-haz`.
+
+For more information, including source model specifications, result tables, and summary report,
+please see the [PEER Probabilistic Seismic Hazard Analysis Code Verification Report -
+2018/03](https://peer.berkeley.edu/sites/default/files/2018_03_hale_final_8.13.18.pdf).
+
+## Implementation Notes
+
+- Most test cases have a version that matches the target results, but that run slowly, as
+  well as a'fast' implementations with coarser discretizations.
+- Set1, Cases 8a, b, and c implementations are the same as the Set1, Case2 'fast' versions.
+- Set2, Case1 (disagreggation) was skipped; implemented in OpenSHA.
+- Set3 tests are not yet implemented.