Add user story template (#325)

.github/ISSUE_TEMPLATE/user_stories.md

@@ -0,0 +1,36 @@
+---
+name: User stories
+about: Suggest new documentation from the perspective of a user!
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+**User story**
+<!--
+A user story is a description of a feature,
+from the perspective of the user who desires it.
+We appreciate all user stories that as long or short,
+simple or detailed, as you like!
+They typically follow a template like the below.
+
+An example:
+
+As an *infrequent user of MDAnalysis*,
+I want to *easily know what the units of MDAnalysis are*,
+so that *I don't accidentally mix up units in my analyses*.
+
+-->
+
+As a *[type of user]*, I want to *[some goal]*,
+so that *[reason why]*.
+
+**Additional context**
+
+<!--
+If there's any current documentation addressing similar issues
+and why it doesn't solve the problem,
+or any other additional context
+-->
+