README: add a build badge (status of the Azure Pipelines build)

Just like so many other OSS projects, we now also have a build badge.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/README.md b/README.md
index f920a42..764c480 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,5 @@
+[![Build Status](https://dev.azure.com/git/git/_apis/build/status/test-git.git)](https://dev.azure.com/git/git/_build/latest?definitionId=2)
+
 Git - fast, scalable, distributed revision control system
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