Issue template - Add "improvement.md"

This is basically the same as the bug-report template, except:

* "Reproduction steps" become "Example use-case"
* "Environment" is removed - because it's not usually a factor. (And if it is,
  it's generally explicit part of the requirements.)

.gitlab/issue_templates/improvement.md

@@ -0,0 +1,20 @@
+Overview
+----------------------------------------
+_Please describe your improvement in detail._
+
+Example use-case
+----------------------------------------
+1. Click on **Contacts -> New Individual**.
+1. Enter **First Name** and **Last Name** and click **Save**.
+
+Current behaviour
+----------------------------------------
+_What is currently possible? What limit ?_
+
+Proposed behaviour
+----------------------------------------
+_What should happen? How is this better? If appropriate/available, include any wireframes or mockups._
+
+Comments
+----------------------------------------
+_Anything else you would like the reviewer to note._