Make draft docs for issue weight feature

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

doc/workflow/README.md

@@ -8,6 +8,7 @@
 - [Importing to GitLab](doc/importing/README.md)
 - [Keyboard shortcuts](shortcuts.md)
 - [Labels](labels.md)
+- [Issue weight](issue_weight.md)
 - [Manage large binaries with git annex](git_annex.md)
 - [Merge Request Approvals](merge_request_approvals.md)
 - [Notification emails](notifications.md)

doc/workflow/issue_weight.md

+# Issue weight
+
+Some issues are bigger than others. It can be time, complexity or value criteria. 
+To distinguish such issues we implemented issue weight attribute. 
+It allows you to set numeric value from 1 to 9 per issue. You can see issue weight on next pages: 
+
+* issue page
+* list of issues
+* Total weight of issues on milestone page
+
+![issue page](issue_weight/issue.png)