Doc: design doc for pdb plugin

docs/design/pdb-plugin.md

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+# Pod Disruption Budget (PDB) Plugin
+
+## Introduction
+
+When users apply jobs to volcano, they may limit the number of Pod copies that are destroyed at the same time. This restriction is often limited by the [PDB resources](https://kubernetes.io/docs/tasks/run-application/configure-pdb/) created by the user. So PDB plugin is provided to meet the PBD constraints set by the user during the Volcano scheduling process.
+
+## Usage
+1. Firstly, please ensure that your k8s version is at least 1.21 and that you have created the PDB resource.
+2. Secondly, user can open `PDB Plugin` via `volcano-scheduler-configmap` in following format:
+
+      ```yaml
+        actions: "reclaim, preempt, shuffle"
+        tiers:
+        - plugins:
+          - name: pdb
+      ```
+*PDB Plugin can be used in any one action of reclaim, preempt, shuffle*
+
+## Solution
+The plugin register , `ReclaimableFn`, `PreemptableFn` and `VictimTasksFn` in `Reclaim`, `Preempt` and `Shuffle` action to filter out tasks that violate the PDB constraint.
+
+The execution flow chart of filter process of `pdb` plugin is shown as below:
+
+![workflow](./images/pdb-workflow.png)
+
+*`DisruptedPods` contains information about pods whose eviction was processed by the API server eviction subresource handler but has not yet been observed by the PodDisruptionBudget controller.*
+
+### About the Cache
+The PDB plugin first registers `informerFactory.Policy().V1().PodDisruptionBudgets()` in `newSchedulerCache()` function.
+
+Then the plugin adds `v1.PodDisruptionBudgetLister` variable to type definition and inits it with `informerFactory.Policy().V1().PodDisruptionBudgets().Lister()` in `New()` function.
+
+Finally the plugin uses this `Lister` to list the PDBs from the informer cache.
+## Feature Interaction
+
+1. By now we only need 1 argument `sla-waiting-time`, so I add it into annotations for simplicity and invocation, but when `sla` plugin is extended with more arguments, a better way to invoke this plugin may be job plugin like `svc` and `ssh`.
+