init: initial beta validate documentaiton

Signed-off-by: Piotr Zaniewski <[email protected]>

content/master/cli/command-reference.md

@@ -320,6 +320,113 @@ flags, options or outputs in future releases.
 Crossplane maintainers may promote or remove commands under `beta` in future 
 releases.
 
+### beta validate
+
+The `crossplane beta validate` validates [compositions]({{<ref "../concepts/compositions>"}}) against provider or XRD schema using
+Kubernetes API server's validation library.
+
+The validate command enables various validation scenarios:
+
+- Validate any Crossplane resource against any schema (provider, XRD).
+- Use the output of the render command to validate function based compositions.
+- When validating resources against provider schemas, the command downloads and
+  cache the providers' CRDs to make future runs much faster.
+- Work with Kubernetes CEL validation rules to enable validation of complex business rules expressed in the composition resources definitions (XRDs).
+- Pass the entire folder with schema files and validate all the Crossplane
+  resources against the schemas.
+
+{{< hint "important" >}}
+The `crossplane beta validate` command performs all validation offline without requiring a running cluster.
+{{< /hint >}}
+
+### Kubernetes custom definition rules
+
+Validate XRD [CEL Rules](https://kubernetes.io/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/#validation-rules)
+
+{{< expand "An example validation rule" >}}
+
+```yaml
+x-kubernetes-validations:
+  - rule: "self.minReplicas <= self.replicas && self.replicas <= self.maxReplicas"
+    message: "replicas should be in between minReplicas and maxReplicas."
+```
+
+{{< /expand >}}
+
+`crossplane beta validate xrd.yaml rong-xr.yaml`
+{{< expand "CEL rule violation" >}}
+
+```console
+[x] CEL validation error example.crossplane.io/v1beta1, Kind=XR, example : spec: Invalid value: "object": replicas should be in between minReplicas and maxReplicas.
+Total 1 resources: 0 missing schemas, 0 success cases, 1 failure cases
+crossplane: error: cannot validate resources: could not validate all resources
+```
+
+{{< /expand >}}
+
+### Validate resource against a schema
+
+In the first scenario we can validate any resource (in this case XR and MR)
+against any schema (in this case provider CRD and an XRD)
+
+`crossplane beta validate schemas.yaml resources.yaml`
+
+The command has two categories of output; _errors_ and _warnings_.
+If there is a validation error or warning, the command will indicate it accordingly. Let's
+try it out, here the command will output validation warning since were are
+missing an XRD:
+
+`crossplane beta validate missing-schemas.yaml resources.yaml`
+
+### Validate render command output
+
+This is a very useful feature when developing functions. Passing the output of
+the _render_ command to the _validate_ command makes sure that our function does
+not violate schema requirements.
+
+`crossplane beta render xr.yaml composition.yaml func.yaml | crossplane beta validate schemas.yaml -`
+
+You might have noticed that there is an error indicating the missing XR. This is
+because functions do not have access to spec fields. The flag _--include-full-xr_ fixed
+it by adding the XR to the _render_ command output and satisfying the validate
+constrains.
+
+`crossplane beta render xr.yaml composition.yaml func.yaml --include-full-xr | crossplane beta validate schemas.yaml -`
+
+### Validate by folder
+
+If you have multiple compositions and resources to validate, it is easy to use
+the _validate_ command to travers all the Crossplane yaml files and validate in
+batch mode across all the resources.
+
+First let's render all our resources into a file (xr, compostion and function).
+It will serve as a base for validation.
+
+`crossplane beta render xr.yaml composition.yaml func.yaml --include-full-xr > rendered.yaml`
+
+The _Extensions_ folder contains packages (provider and configuration) to validate against `tree Extensions`
+
+`crossplane beta validate Extensions rendered.yaml`
+
+After executing this command you will notice that the validate command downloads
+missing CRDs into a _.crossplane_ folder. This enables the validation process
+without the need to access external cluster!
+
+Running the same command again will not trigger the download since we have
+cached all the schemas.
+
+`crossplane beta validate Extensions rendered.yaml`
+
+Finally we can use a custom directory as a cache for downloading the schemas.
+This enables having a centralized and shared cache directory between different
+projects.
+
+`crossplane beta validate Extensions rendered.yaml --cache-dir mycache`
+
+Or clean the cache directory.
+
+`crossplane beta validate Extensions rendered.yaml --cache-dir mycache --clean-cache`
+
 ### beta render 
 
 The `crossplane beta render` command previews the output of a