HADOOP-14196. Azure Data Lake doc is missing required config entry. Contributed by Atul Sikaria

hadoop-tools/hadoop-azure-datalake/src/site/markdown/index.md

@@ -132,6 +132,11 @@ service associated with the client id. See [*Active Directory Library For Java*]
 Add the following properties to your `core-site.xml`
 
 ```xml
+<property>
+  <name>dfs.adls.oauth2.access.token.provider.type</name>
+  <value>ClientCredential</value>
+</property>
+
 <property>
   <name>dfs.adls.oauth2.refresh.url</name>
   <value>TOKEN ENDPOINT FROM STEP 7 ABOVE</value>