Fix git-subtree install instructions
Fix git-subtree install instructions

Update the install instructions to reflect the changes for an
integrated git-subtree.

Signed-off-by: David A. Greene <699889fa4d775c82b36574aaa0f8ebb772e0c366@obbligato.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git