Added documentation for tags

doc/index.markdown

@@ -46,6 +46,7 @@ Using Kanboard
 - [Subtasks](subtasks.markdown)
 - [Analytics for tasks](analytics-tasks.markdown)
 - [User mentions](user-mentions.markdown)
+- [Tags](tags.markdown)
 
 ### Working with users and groups
 

doc/tags.markdown

@@ -0,0 +1,24 @@
+Tags
+====
+
+With Kanboard, you can associate one or many tags to a task.
+You can define tags globally for all projects or only for a specific project.
+
+![Tags on the board](screenshots/tags-board.png)
+
+From the task form, you can enter the desired tags:
+
+![Tags form](screenshots/tags-task.png)
+
+The auto-completion form will show up to suggest available tags.
+
+You can also create tags directly from the task form.
+By default, when you create tags from a task form they are associated to the current project:
+
+![Project Tags](screenshots/tags-projects.png)
+
+You can also create these tags manually in the project settings.
+
+To define tags globally for all projects, go to the application settings:
+
+![Global Tags](screenshots/tags-global.png)