Add release notes documentation

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

doc/workflow/README.md

@@ -13,5 +13,6 @@
 - [Project users](add-user/add-user.md)
 - [Protected branches](protected_branches.md)
 - [Web Editor](web_editor.md)
+- [Releases](releases.md)
 - [Merge Requests](merge_requests.md)
 - ["Work In Progress" Merge Requests](wip_merge_requests.md)

doc/workflow/releases.md

+# Releases
+
+You can turn any git tag into release by just adding a release notes to it. 
+Release notes behaves like any other markdown form in GitLab so you can write text and drag-n-drop files to it.
+Release notes are stored in GitLab database. 
+
+There are several ways to add release notes: 
+
+* In UI when you create new git tag with GitLab
+* In UI when you add release notes to existing git tag
+* with GitLab API
+
+## New tag page with release notes text area
+
+![new_tag](releases/new_tag.png)
+
+## Tags page with button to add or edit release notes for existing git tag
+
+![tags](releases/tags.png)
+