新增learn-kubernetes（https://github.com/yyong-brs/learn-kubernetes）相关文件

learn/learn-kubernetes-master/kiamol/ch20/todo-custom/items/todo-ch20.yaml

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+apiVersion: "ch20.kiamol.net/v1"
+kind: ToDo
+metadata:
+  name: ch20
+  labels:
+    kiamol: ch20
+spec:
+  item: "Finish KIAMOL Ch20"
+  dueDate: "2020-07-26"

learn/learn-kubernetes-master/kiamol/ch20/todo-custom/items/todo-ch21.yaml

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+apiVersion: "ch20.kiamol.net/v1"
+kind: ToDo
+metadata:
+  name: ch21
+  labels:
+    kiamol: ch20
+spec:
+  item: "Plan KIAMOL Ch21"
+  dueDate: "2020-07-27"

learn/learn-kubernetes-master/kiamol/ch20/todo-custom/todo-crd.yaml

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+apiVersion: apiextensions.k8s.io/v1  # minimum 1.16
+kind: CustomResourceDefinition
+metadata:
+  name: todos.ch20.kiamol.net
+  labels:
+    kiamol: ch20
+spec:
+  group: ch20.kiamol.net
+  scope: Namespaced
+  names:
+    plural: todos
+    singular: todo
+    kind: ToDo
+    shortNames:
+    - td
+  versions:
+    - name: v1
+      served: true
+      storage: true
+      schema:
+        openAPIV3Schema:
+          type: object
+          properties:
+            spec:
+              type: object
+              properties:
+                item:
+                  type: string
+                dueDate:
+                  type: string
+                  format: date 

learn/learn-kubernetes-master/kiamol/ch20/todo-custom/update/todo-crd-with-columns.yaml

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+apiVersion: apiextensions.k8s.io/v1
+kind: CustomResourceDefinition
+metadata:
+  name: todos.ch20.kiamol.net
+  labels:
+    kiamol: ch20
+spec:
+  group: ch20.kiamol.net
+  scope: Namespaced
+  names:
+    plural: todos
+    singular: todo
+    kind: ToDo
+    shortNames:
+    - td
+  versions:
+    - name: v1
+      served: true
+      storage: true
+      schema:
+        openAPIV3Schema:
+          type: object
+          properties:
+            spec:
+              type: object
+              properties:
+                item:
+                  type: string
+                dueDate:
+                  type: string
+                  format: date  
+      additionalPrinterColumns:
+        - name: Item
+          type: string
+          description: What to do
+          jsonPath: .spec.item
+        - name: Due
+          type: string
+          description: When to do it
+          jsonPath: .spec.dueDate