additional stable curst fault notes

stable-crust/fault/README.md

+# Stable Crust Fault Sources
+
+## Implementation note: Rupture properties
+
+Fault sources in the stable crust tectonic setting of the central and eastern
+U.S. are commonly constructed by stitching different groups of fault sections together. Under the
+current implementation, properties of a rupture are governed by the first section defined in a
+rupture-set. So, for example, the <90° dips of the Meeman-Selby section of the eastern Rift Margin
+(south) and the the Reelfoot fault sections of the USGS and SSCn New Madrid models are commonly
+overriden to be vertical faults. See also `nshmp-haz` notes on
+[fault section stitching](https://code.usgs.gov/ghsc/nshmp/nshmp-haz/-/blob/main/docs/pages/Source-Types.md#fault-section-stitching).
+
+## Implementation note: Rupture floating
+
+Also note that magnitude scaling relations do not play a role in the modeling of stable-crust
+ruptures. Although an assigned MFD magnitude would realistically be somewhat smaller than the
+modeled rupture (if one were using a scaling relation), the rupture fills the full fault surface.