Add priority label explanation in the guidebook.

Closes #53 (closed)

docs/workflow/gitlab.md

@@ -295,6 +295,23 @@ issues which relate to a particular role within a product. It would not be
 necessary to label issues with, for example, the epic they are part of because
 one could just list all issues in that epic directly.
 
+### Priority labels
+
+These labels describe priorities and indicate in which **order** issues should be
+**done** in the sprint, e.g. priority 1 tasks should be done before priority 2, 
+and so on.
+
+* **Priority 1**. Denotes that an issue that is either **urgent** or that needs to get
+  done as soon as possible with top priority.
+* **Priority 2**. Denotes that an issue with a "normal" priority. An issue that can be
+  scheduled in a normal Sprint and get done as part of the Sprint at any time
+  after **Priority 1** issues have all been done.
+* **Priority 3**. Denotes that we would like to get that issue done in a given Sprint
+  but that is not necessarly needed for that Sprint in particular. It normally has
+  a date when it is required to get done by but that fall outside the sprint end date. 
+* **Priority 4**. Denotes that an issue is a *nice to have* but do not have any pressure
+  on time, when it should get done. Could be improvements that it will be good to have.
+
 ### Role labels
 
 We have some labels which describe which role(s) within the product team