docs: added docs for Jenkins pipeline (#1527)

* Added docs for Jenkins

* Added index

www/docs/ci/jenkins.md

@@ -0,0 +1,37 @@
+# Jenkins
+
+Here is how to set up a [Jenkins](https://www.jenkins.io/) pipeline using Jenkinsfile:
+
+```groovy
+pipeline {
+  ...
+
+  stages {
+    stage('Compile') {
+      steps {
+        sh 'go build'
+      }
+    }
+
+    stage('Test') {
+      steps {
+        sh 'go test ./...'
+      }
+    }
+
+    stage ('Release') {
+      when {
+        buildingTag()
+      }
+
+      environment {
+        GITHUB_TOKEN = credentials('github-token')
+      }
+
+      steps {
+        sh 'curl -sL https://git.io/goreleaser | bash'
+      }
+    }
+  }
+}
+```

www/mkdocs.yml

@@ -54,6 +54,7 @@ nav:
   - ci/codefresh.md
   - ci/drone.md
   - ci/semaphore.md
+  - ci/jenkins.md
 - Customization:
   - About: customization/index.md
   - customization/project.md