Update git submodules

* Update docs/submodules/qtip from branch 'stable/danube'
  - Merge "Add overview in developer guide" into stable/danube
  - Add overview in developer guide

    Change-Id: I491f921d87cf0cec3bbef5a78e9f3065b1e012a1
    Signed-off-by: Yujun Zhang <zhang.yujunz@zte.com.cn>
    (cherry picked from commit cb8ec13d8981cea1dc5556d86ea01b45c29f1046)

diff --git a/docs/submodules/qtip b/docs/submodules/qtip
index 0954c5f..4dc621a 160000 (submodule)
--- a/docs/submodules/qtip
+++ b/docs/submodules/qtip
@@ -1 +1 @@
-Subproject commit 0954c5f7ae1f18d07662d578eaf015225c5fb1e7
+Subproject commit 4dc621a70a4fcf8e6b2690950be7162fb003cf75