Add external for documentation images

Author: billsacks

.gitignore

@@ -5,6 +5,7 @@ manage_externals.log
 /tools/PTCLM/
 /cime/
 /components/
+/doc/source/images/
 
 # ignore svn directories
 **/.svn/**

Externals_CLM.cfg

@@ -12,6 +12,13 @@ repo_url = https://github.com/ESCOMP/PTCLM
 tag = PTCLM2_20200121
 required = True
 
+[doc-images]
+local_path = doc/source/images
+protocol = git
+repo_url = https://github.com/ESCOMP/CTSM-doc-images.git
+hash = 0f039e7a043fabfce91ddf4f41186a3014603aac
+required = False
+
 [externals_description]
 schema_version = 1.0.0