Add checking policies to contribution checklist

Whilst we call out specific policies in the contribution checklist, we
should make sure that any contributions made to Apertis do not violate any
of Apertis' policies. Explicitly add this to the contribution checklist.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

content/policies/contribution_checklist.md

@@ -85,6 +85,13 @@ out per contribution type:
   by any proposed changes. Ensure that such value can be expected before
   starting development.
 
+- `Contributor` **Ensure proposed changes comply with all existing relevant
+  policies**: The Apertis maintainers will be evaluating whether the proposed
+  changes comply with the full range of policies which govern the Apertis
+  project. Now is a good time, before significant effort is expended, to check
+  that the proposed changes align with the Apertis projects policies to guide
+  development and avoid disappointment or wasted effort.
+
 ## Concept Design
 
 - `Contributor` **Survey the state of art**: The project strives to adapt and
@@ -239,6 +246,9 @@ out per contribution type:
   taken to evaluate that platform specific changes are made in a way that other
   platforms are not negatively impacted.
 
+- `Maintainer` **Ensure adherence to Apertis policies**: Changes to Apertis
+  should only be accepted if they adhere to all the relevant Apertis policies.
+
 ## Code
 
 - `Maintainer` **Check coding conventions**: