Added the contribution guide notice

This is shown at the creation of new issues and new merge requests, when
the repository has a contribution guide file.

app/models/repository.rb

@@ -134,6 +134,7 @@ class Repository
     Rails.cache.delete(cache_key(:commit_count))
     Rails.cache.delete(cache_key(:graph_log))
     Rails.cache.delete(cache_key(:readme))
+    Rails.cache.delete(cache_key(:contribution_guide))
   end
   def graph_log
@@ -167,6 +168,12 @@ class Repository
     end
   end
+  def contribution_guide
+    Rails.cache.fetch(cache_key(:contribution_guide)) do
+      tree(:head).contribution_guide
+    end
+  end
+
   def head_commit
     commit(self.root_ref)
   end

app/models/tree.rb

 class Tree
-  attr_accessor :entries, :readme
+  attr_accessor :entries, :readme, :contribution_guide
   def initialize(repository, sha, path = '/')
     path = '/' if path.blank?
@@ -10,6 +10,11 @@ class Tree
       readme_path = path == '/' ? readme_tree.name : File.join(path, readme_tree.name)
       @readme = Gitlab::Git::Blob.find(git_repo, sha, readme_path)
     end
+
+    if contribution_tree = @entries.find(&:contribution?)
+      contribution_path = path == '/' ? contribution_tree.name : File.join(path, contribution_tree.name)
+      @contribution_guide = Gitlab::Git::Blob.find(git_repo, sha, contribution_path)
+    end
   end
   def trees

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

 %div.issue-form-holder
   %h3.page-title= @issue.new_record? ? "New Issue" : "Edit Issue ##{@issue.iid}"
   %hr
+  - if @repository.contribution_guide && !@issue.persisted?
+    - contribution_guide_url = project_blob_path(@project, tree_join(@repository.root_ref, @repository.contribution_guide.name))
+    .alert.alert-info.col-sm-10.col-sm-offset-2
+      ="Please review the <strong>#{link_to "guidelines for contribution", contribution_guide_url}</strong> to this repository.".html_safe
   = form_for [@project, @issue], html: { class: 'form-horizontal issue-form' } do |f|
     -if @issue.errors.any?
       .alert.alert-danger

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

+- if @repository.contribution_guide && !@merge_request.persisted?
+  - contribution_guide_url = project_blob_path(@project, tree_join(@repository.root_ref, @repository.contribution_guide.name))
+  .alert.alert-info.col-sm-10.col-sm-offset-2
+    ="Please review the <strong>#{link_to "guidelines for contribution", contribution_guide_url}</strong> to this repository.".html_safe
 = form_for [@project, @merge_request], html: { class: "merge-request-form form-horizontal" } do |f|
   -if @merge_request.errors.any?
     .alert.alert-danger