Add wxWidgets as a submodule

ext/README.txt

@@ -0,0 +1,30 @@
+What is this?
+=============
+
+This folder holds the source for external projects used by Phoenix, (currently
+just wxWidgets) as git submodules.  This allows Phoenix to use a specific
+revision of the code in the other projects and not depend on the developer
+fetching the correct version of the code on their own.
+
+When you first checkout the Phoenix source using git you will need to tell git
+to also fetch the submodules, like this:
+
+    git submodule init
+    git submodule update
+
+To learn more about git submodules, please see the following:
+
+    http://git-scm.com/book/en/v2/Git-Tools-Submodules
+    http://blogs.atlassian.com/2013/03/git-submodules-workflows-tips/
+    http://www.speirs.org/blog/2009/5/11/understanding-git-submodules.html
+    
+
+Notes to self
+=============
+
+To use a different repo URL for a submodule than what others will see (in
+order to make and test local changes to the submodule) then you can set that
+with a command like:
+
+    git config submodule.MODULE_NAME.url PRIVATE_URL
+