Ensure git does not modify manifests

Ensure git does NOT modify manifests to prevent hash mismatch.

Without this change one would get the following error on running
setup.exe deployed over the web:

"Application manifest has either a different computed hash than the one
specified or no hash specified at all"

Author: TheCodeArtist

.gitattributes

@@ -15,3 +15,7 @@
 *.PDF	 diff=astextplain
 *.rtf	 diff=astextplain
 *.RTF	 diff=astextplain
+
+*.vsto		 binary
+*.deploy	 binary
+*.manifest	 binary