manifest: add a general github remote

Just makes it a bit easier to track personal repos and those from themuppets, etc
in local_manifests.

Change-Id: I05105b94288e922470c3c57d442867600ba10fe3

default.xml

@@ -4,6 +4,9 @@
   <remote  name="xda"
            fetch=".." />
 
+  <remote  name="github"
+           fetch="https://github.com/" />
+
   <remote  name="aosp"
            fetch="https://android.googlesource.com"
            review="android-review.googlesource.com" />