add source.properties file to the offline docs build
which is required to get this in the SDK Manager

Change-Id: If53cdedcb8f5910fe37add44168348c9fd14d0cb
diff --git a/docs/source.properties b/docs/source.properties
new file mode 100644
index 0000000..77a760b
--- /dev/null
+++ b/docs/source.properties
@@ -0,0 +1,3 @@
+Pkg.Revision=24.0
+Pkg.Desc=Android offline API reference
+Pkg.Path=docs
\ No newline at end of file