add mobile submodule

.gitmodules

+[submodule "datavis-mobile"]
+	path = datavis-mobile
+	url = https://github.com/probably-not-porter/datavis-mobile

datavis-mobile @ 4708a262

+Subproject commit 4708a2624bdfa2a432571f37d9179e66f548db50

docs/MOBILE.md

@@ -2,7 +2,7 @@
 
 [[ Main README ]](../README.md)
 
-The purpose of this document is to support the datavis-mobile android application.
+The purpose of this document is to support the `datavis-mobile` android application, which is a `submodule` of this repository. The original repository for `datavis-mobile` can be found [here](https://github.com/probably-not-porter/datavis-mobile).
 
 ## Version Information