Update jenkins.md docs, for GitTools#313

Author: serra

docs/more-info/build-server-setup/jenkins.md

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+# Jenkins Setup
+
+Injecting environment variables is not supported in Jenkins natively, but Jenkins plugins exist that provide this functionality. Of these plugins [EnvInject] appears to be the most popular with over 20k downloads per month.
+
+To inject the GitVersion variables as environment variables for a build job using  [EnvInject], do the following:
+
+ 1. Add a **Execute Windows batch command** build step with command:
+    `gitversion /output props > gitversion.properties`
+ 1. Add a **Inject environment variables** build step and use value 'gitversion.properties" for the "Properties File Path" parameter 
+
+You can verify correct inject of environment variables by adding another "Execute Windows batch command" build step with the following command:
+
+```
+@echo Retrieving some GitVersion environment variables:
+@echo %gitversion.SemVer%
+@echo %gitversion.BranchName%
+@echo %gitversion.AssemblySemVer%
+@echo %gitversion.MajorMinorPatch%
+@echo %gitversion.Sha%
+```
+
+  [EnvInject]: https://wiki.jenkins-ci.org/display/JENKINS/EnvInject+Plugin