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: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

README.md

+[![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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