Mention the extension in the TFS Build vNext instructions

Author: Pascal Berger

docs/build-server-support/build-server/tfs-build-vnext.md

@@ -1,6 +1,6 @@
 # Team Foundation Build (vNext) Setup
 ## Basic Usage
-In Team Foundation Build (the web based build system) you can call GitVersion either using the Command Line build step or install a custom build step. The custom build step requires a one-time setup to import the GitVersion task into your TFS or VSO instance.
+In Team Foundation Build (the web based build system) you can call GitVersion either using the Command Line build step or install an extension / custom build step. The custom build step requires a one-time setup to import the GitVersion task into your TFS or VSO instance.
 
 ## Executing GitVersion
 ### Using GitVersion with the MSBuild Task NuGet Package
@@ -17,7 +17,13 @@ See [MSBuild Task](/usage/msbuild-task) for further instructions how to use the
 6. If you want to update the build number you need to send a [logging command](https://github.com/Microsoft/vso-agent-tasks/blob/master/docs/authoring/commands.md) to TFS.
 
 ### Using the custom GitVersion build step
-#### Installing/updating the custom build step
+#### Installing
+##### Installing the extension
+For Visual Studio Team Service or TFS 2015 Update 2 or higher it is recommonded to install the GitVersion extension:
+1. Install the [GitVersion Extension](https://marketplace.visualstudio.com/items?itemName=gittools.gitversion).
+
+##### Manually installing/updating the custom build step
+If you run TFS 2015 RTM or Update 1 or don't want to install the GitVersion extension you can install the build task manually:
 1. Install the `tfx` command line tool as shown [here](https://github.com/Microsoft/tfs-cli/blob/master/README.md#install).
 2. For TFS 2015 On-Prem configure Basic Authentication in TFS as shown [here](https://github.com/Microsoft/tfs-cli/blob/master/docs/configureBasicAuth.md).
 3. Download the GitVersion TFS build task from the latest release on the [GitVersion releases page](https://github.com/GitTools/GitVersion/releases) and unzip.
@@ -56,6 +62,7 @@ See [Variables](/more-info/variables/) for more info on the variables.
 
 #### Known limitations
 * If you are using on premises TFS, make sure you are using at least **TFS 2015 Update 1**, otherwise a few things will not work.
+* Installing the extension on an on premise TFS requires at least TFS 2015 Update 2.
 * You need to make sure that all tags are fetched for the Git repository, otherwise you may end with wrong versions (e.g. `FullSemVer` like `1.2.0+5` instead of `1.2.0` for tagged releases) 
 Just checking the `Clean Repository` check box in the build definition settings might not be enough since this will run a `git clean -fdx/reset --hard` without fetching all tags later. 
 You can force deletion of the whole folder and a re-clone containing all tags by settings the variable `Build.Clean` to `all`.