docs: add doc for CRD upgrade (#484)

Signed-off-by: Niladri Halder <niladri.halder26@gmail.com>

docs/troubleshooting/crd-upgrade.md

@@ -0,0 +1,30 @@
+# CRD Upgrade
+
+The following CRDs were upgraded in v2.3.0 to support 'zstd' ZFS pool compression variants:
+- zfsvolumes.zfs.openebs.io
+- zfssnapshots.zfs.openebs.io
+- zfsrestores.zfs.openebs.io
+
+Using zstd-3 or similar compression algorithms (zstd with the '-x' suffix) through the StorageClass may lead to a failure when provisioning a volume, if the CRDs are not upgraded.
+
+Error log when trying to use 'zstd-3' without upgrading CRDs:
+```
+zfs.csi.openebs.io_openebs-zfslocalpv-zfs-localpv-controller-0_2332b051-644b-485c-80e1-be9aed3a5827  failed to provision volume with StorageClass "openebs-zfspv": rpc error: code = Internal desc = not able to provision the volume, nodes [node-0-125210 node-1-125210 node-2-125210], err : ZFSVolume.zfs.openebs.io "pvc-c9f7ad46-9efb-4e2a-87f9-149a0d1cacae" is invalid: spec.compression: Invalid value: "zstd-3": spec.compression in body should match '^(on|off|lzjb|zstd|gzip|gzip-[1-9]|zle|lz4)$'
+```
+
+Upgrade the CRDs using the following commands:
+```
+# Upgrade ZFSVolumes CRD
+curl -LO https://raw.githubusercontent.com/openebs/zfs-localpv/zfs-localpv-2.3.1/deploy/yamls/zfsvolume-crd.yaml
+kubectl patch crd zfsvolumes.zfs.openebs.io --patch-file zfsvolume-crd.yaml
+
+# Upgrade ZFSSnapshots CRD
+curl -LO https://raw.githubusercontent.com/openebs/zfs-localpv/zfs-localpv-2.3.1/deploy/yamls/zfssnapshot-crd.yaml
+kubectl patch crd zfssnapshots.zfs.openebs.io --patch-file zfssnapshot-crd.yaml
+
+# Upgrade ZFSRestores CRD
+curl -LO https://raw.githubusercontent.com/openebs/zfs-localpv/zfs-localpv-2.3.1/deploy/yamls/zfsrestore-crd.yaml
+kubectl patch crd zfsrestores.zfs.openebs.io --patch-file zfsrestore-crd.yaml
+```
+
+Delete and recreate the PVC.