review REQUIRED.md files

minor MD lint fixes, add placeholder code.json.

TODO: code.json must be updated with version/release tag and DOI. see ghsc/nshmp/nshm-hawaii#1

See ghsc/nshmp/nshmp-haz-v2#464

volcanic/grid-sources/README.md

@@ -4,6 +4,10 @@ Gridded (and smoothed) seismicity sources are modeled as uniformely distributed
 
 All sub-catalogs were declusterd using nearest-neighbor (`nn`) and Reasenberg (1985; `r85`) algorithms prior to smoothing earthquake rates using a single, adaptive kernel approach.
 
+## Gridded rates
+
+Rates of M = 5.0 earthquakes, `n(m=5)`, given in Table 1 of Petersen et al. (2014) are the Gutenberg-Richter rate of magnitude 5.0 earthquakes derived from the total gridded a-value multiplied by a scale factor for the catalog time period and declustering method. These `n(m=5)` rates are converted to a-values (rate of earthquakes of magnitude 0.0) using ![formula](https://render.githubusercontent.com/render/math?math=n(m)\=10^{a-bm}) for use in grid-source `rate-tree.json` files. Spatial PDFs were created by normalizing the 1960-2019 catalog a-grid file.
+
 ## Grid source rupture set names and IDs
 
 Region     | Depth           | PDF¹ | ID