Add new atmospheric_physics section with directory structure page.

Author: nusbaume

docs/atmospheric_physics/design/directory-structure.md

@@ -0,0 +1,35 @@
+# atmospheric_physics directory structure
+
+This page lists out the directory structure for the atmospheric_physics repo, and what the general purpose of each directory and subdirectory is.
+
+## Top-level directories
+
+These directories represent code and tools that is solely contained within the atmospheric_physics repository.  If there is a sub-directory that is also an external submodule or repository it will be marked with the "(external)" label.
+
+### **doc/**
+
+Contains files used to document the current state of the atmospheric_physics repository, such as a `ChangeLog` and the `NamesNotInDictionary.txt` file which lists all standard names that are currently  not in the official [CCPP Standard Names repo](https://github.com/ESCOMP/CCPPStandardNames).
+
+### **schemes/**
+
+Contains subdirectories for all of the CCPP-ized physics schemes, with the subdirectories containing the core physics scheme source code,
+the associated CCPP metadata files, namelist XML files, and any relevant depdendency files.
+
+Note that there is a special `to_be_ccppized` subdirectory which contains source code that is needed by certain CCPP physics schemes, but which is not
+yet fully CCPP compliant (e.g. may have code that is host-model specific).
+
+### **suites/**
+
+Contains CCPP Suite Definition Files (SDFs) for official scientific and production configurations of the CAM-SIMA host model.
+
+### **test/**
+
+Contains code and tools used to run tests on the CCPP-ized physics schemes.
+
+**Subdirectories**:
+
+- cmake   - Contains cmake files needed to configure and build  unit tests.
+- docker  - Contains dockerfiles needed to configure, butild, and run unit tests.
+- include - Contains utility source code needed to build and run unit tests.
+- musica  - Contains test code for the CCPP-ized [Multie-Scale Infrastructure for Chemistry Modeling (MUSICA)](https://github.com/NCAR/musica) components.
+- test_suites - Contains test SDFs for use in CAM-SIMA snapshot regression testing.

mkdocs.yml

@@ -32,6 +32,8 @@ nav:
   - Usage:
     - usage/creating-a-case.md
     - usage/history.md
+  - Atmospheric_physics:
+    - directory-structure.md
 
 plugins:
   - search