Add LSTM submodule to ngen repository in extern/lstm

.gitmodules

@@ -43,3 +43,6 @@
 [submodule "extern/netcdf-cxx4"]
 	path = extern/netcdf-cxx4/netcdf-cxx4
 	url = https://github.com/Unidata/netcdf-cxx4/
+[submodule "extern/lstm/lstm"]
+	path = extern/lstm/lstm
+	url = https://github.com/CIROH-UA/lstm

extern/lstm/README.md

@@ -0,0 +1,27 @@
+# LSTM Submodule
+
+## About
+This submodule is linked in from: https://github.com/CIROH-UA/lstm, which is a fork of https://github.com/NOAA-OWP/lstm. This directory follows the template for linking submodules from https://github.com/NOAA-OWP/ngen/edit/master/extern/cfe/.
+
+#### Extra Outer Directory
+
+Currently there are two directory layers beneath the top-level *extern/* directory.  This was done so that certain things used by NGen (i.e., a *CMakeLists.txt* file for building shared library files) can be placed alongside, but not within, the submodule.
+
+## Working with the Submodule
+
+Some simple explanations of several command actions are included below.  To better understand what these things are doing, consult the [Git Submodule documentation](https://git-scm.com/book/en/v2/Git-Tools-Submodules).
+
+### Getting the Latest Changes
+
+There are two steps to getting upstream submodule changes fully 
+  1. fetching and locally checking out the changes from the remote
+  2. committing the new checkout revision for the submodule
+
+To fetch and check out the latest revision (for the [currently used branch](#viewing-the-current-branch)):
+
+    git submodule update --init -- extern/lstm/lstm
+
+To commit the current submodule checkout revision to the CIROH UA NGen repo:
+
+    git add extern/lstm/lstm
+    git commit