Added notes about disabling of Node 6 warning message (#15534)

* Added notes about disabling

* Update docs/migrateNode10.md

Co-authored-by: Alexander Smolyakov <[email protected]>

* Updated table of content

* Fixed typo

Co-authored-by: Alexander Smolyakov <[email protected]>

Author: Anatoly Bolshakov

docs/migrateNode10.md

@@ -3,6 +3,7 @@
   - [Common packages dependent on `azure-pipeline-task-lib`](#common-packages-dependent-on-azure-pipeline-task-lib)
 - [List of known dependency issues](#list-of-known-dependency-issues)
   - [`fs` module](#fs-module)
+- [How to disable warnings for Node 6 deprecation](#how-to-disable-warnings-for-node-6-deprecation)
 
 # Upgrading Tasks to Node 10
 
@@ -111,4 +112,9 @@ The following `fs` functions all have incompatibilities. In addition, any other
 - fs.unlink
 - fs.utimes
 - fs.write
-- fs.writeFile
+- fs.writeFile
+
+# How to disable warnings for Node 6 deprecation
+To avoid noise in pipeline logs - you can disable Node 6 deprecation warnings by setting up DISABLE_NODE6_DEPRECATION_WARNING agent knob in one of 2 ways:
+- Set up `DISABLE_NODE6_DEPRECATION_WARNING` pipeline variable as `true`
+- For self-hosted agents - set up environment variable `DISABLE_NODE6_DEPRECATION_WARNING` as `true`