fix(docs): Docs on upgrading from 2.7 - 2.8 (#6143)

* Docs on upgrading from 2.7 - 2.8

* Wording update

docs-gb/upgrading.md

@@ -1,5 +1,14 @@
 # Upgrading
 
+## Upgrading from 2.7 - 2.8
+
+Core 2.8 introduces several new fields in our CRDs:
+* `statefulSetPersistentVolumeClaimRetentionPolicy` enables users to configure the cleaning of PVC on their **servers**. This field is set to **retain** as default.
+* `Status.selector` was introduced as a mandatory field for **models** in 2.8.4 and made optional in 2.8.5. This field enables autoscaling with HPA.
+* `PodSpec` in the `OverrideSpec` for **SeldonRuntimes** enables users to customize how Seldon Core 2 pods are created. In particular, this also allows for setting custom taints/tolerations, adding additional containers to our pods, configuring custom security settings.
+
+These added fields do not result in breaking changes, apart from 2.8.4 which required the setting of the `Status.selector` upon upgrading. This field was however changed to optional in the subsequent 2.8.5 release. Updating the CRDs (e.g. via helm) will enable users to benefit from the associated functionality.
+
 ## Upgrading from 2.6 - 2.7
 
 All pods provisioned through the operator i.e. `SeldonRuntime` and `Server` resources now have the