Update readme

README.md

@@ -53,10 +53,21 @@
 In your `.bashrc` or `.tcshrc` or other rc file add a line:
 
 ##### NCCS
+
+NCCS currently has two different OSs. So you'll need to use different modulefiles depending on which OS you are using.
+
+###### SLES 12
+
 ```
 module use -a /discover/swdev/gmao_SIteam/modulefiles-SLES12
 ```
 
+###### SLES 15
+
+```
+module use -a /discover/swdev/gmao_SIteam/modulefiles-SLES15
+```
+
 ##### NAS
 ```
 module use -a /nobackup/gmao_SIteam/modulefiles
@@ -148,6 +159,15 @@ you choose.
 
 If all you wish is to build the model, you can run `parallel_build.csh` from a head node. Doing so will checkout all the external repositories of the model and build it. When done, the resulting model build will be found in `build/` and the installation will be found in `install/` with setup scripts like `gcm_setup` and `fvsetup` in `install/bin`.
 
+#### Building at NCCS (Multiple OSs)
+
+In all the examples below, NCCS builds will act differently. Because NCCS currently has two different OSs, when you use
+`parallel_build.csh` you will see that the `build` and `install` directories will be appended with `-SLES12` or `-SLES15` depending
+on where you submitted to. When NCCS moves to a single OS again, this will be removed.
+
+Note that if you use the `-builddir` and `-installdir` options, you can override this behavior and no OS will be automatically
+appended.
+
 #### Develop Version of GEOS GCM
 
 `parallel_build.csh` provides a special flag for checking out the