Adds explanation of our review label system.

CONTRIBUTING.md

@@ -21,6 +21,16 @@ _This notice should stay as the first item in the CONTRIBUTING.md file._
 
 The following contents has to be considered as an extension over [gitlab-ce contributing guidelines](https://docs.gitlab.com/ce/development/contributing/index.html).
 
+### How we prioritise MRs from the wider community
+
+Currently we use a system of scoped labels to help us priortise which MRs our team will review. 
+
+| Label | Meaning | Use Cases |
+| ---- | ----- | ----- |
+| ~"Review::P1" | Highest priority to review | Indicates an MR that might solve an urgent pain point for users, contributes to the strategic direction of the runner development as laid out by the Product team, or fixes a critical issue. We maintain a hard cap on the number of contributions marked ~"Review::P1" at which is currently set at 3|
+| ~"Review::P2" | Default priority to review | All incoming MRs should be defaulted to this |
+| ~"Review::P3" | Lowest priority to review | WIP MRs, feature enhancements that only appear to serve a small number of users |
+
 ### Contributing new [executors](https://docs.gitlab.com/runner/#selecting-the-executor)
 
 We are no longer accepting or developing new executors for a few
@@ -37,7 +47,7 @@ With GitLab 12.1, we introduced the [custom
 executor](https://gitlab.com/gitlab-org/gitlab-runner/issues/2885),
 which will provide a way to create an executor of choice.
 
-## Workflow lables
+## Workflow labels
 
 We have some additional labels plus those defined in [gitlab-ce workflow labels](https://docs.gitlab.com/ce/development/contributing/issue_workflow.html)
 
@@ -45,3 +55,4 @@ We have some additional labels plus those defined in [gitlab-ce workflow labels]
 - OS: ~"os::Linux" ~"os::MacOSX" ~"os:FreeBSD" ~"os::Windows" 
 - executor: ~"executor::docker" ~"executor::kubernetes" ~"executor::docker\-machine" ~"executor::docker\-machine" ~"executor::shell" ~"executor::parallels" ~"executor::virtualbox" 
 
+