Add help section for services

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/project_services/jenkins_service.rb

@@ -41,6 +41,10 @@ class JenkinsService < CiService
     'An extendable open source continuous integration server'
   end
 
+  def help
+    'You must have installed GitLab Hook plugin into Jenkins.'
+  end
+
   def to_param
     'jenkins'
   end

app/models/service.rb

@@ -45,6 +45,10 @@ class Service < ActiveRecord::Base
     # implement inside child
   end
 
+  def help
+    # implement inside child
+  end
+
   def to_param
     # implement inside child
   end

app/views/projects/services/_form.html.haml

@@ -18,6 +18,10 @@
           %li= msg
 
 
+  - if @service.help.present?
+    .bs-callout
+      = @service.help
+
   .form-group
     = f.label :active, "Active", class: "control-label"
     .col-sm-10