README: Add documentation build status from Azure Pipelines [ci skip]

README.md

@@ -5,6 +5,7 @@ GDAL - Geospatial Data Abstraction Library
 [![Build status](https://ci.appveyor.com/api/projects/status/jtwx0pcr0y01i17p/branch/master?svg=true)](https://ci.appveyor.com/project/OSGeo/gdal)
 [![Build Status](https://scan.coverity.com/projects/749/badge.svg?flat=1)](https://scan.coverity.com/projects/gdal)
 [gdalautotest coverage](https://gdalautotest-coverage-results.github.io/coverage_html/index.html)
+[![Documentation build Status](https://dev.azure.com/osgeo/gdal/_apis/build/status/OSGeo.gdal.doc?branchName=master&jobName=Documentation)](https://dev.azure.com/osgeo/gdal/_build/latest?definitionId=2&branchName=master&jobName=Documentation)
 
 | Tested configurations    |
 | ------------------------ |