Add README

Author: sbernauer

crates/stackable-crd-previewer/README.md

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+# stackable-crd-previewer
+
+The purpose of this crate is to preview the effects of code changes on our CRDs.
+
+Use the following command to re-generate the CRDs:
+
+```bash
+cargo check -p stackable-crd-previewer
+```
+
+This should implicitly happen by `rust-analyzer` or `cargo check`, so shouldn't be needed to invoke
+explicitly normally.
+
+With an existing Kubernetes context you can run the following command to check if the CRDs are valid:
+
+```bash
+kubectl apply -f generated-crd-previews/ --dry-run=server
+```