doc: add documents for admission picker

Author: ssfdust

docs/05-design/02-admission-picker.md

@@ -0,0 +1,21 @@
+# Admission Picker
+
+## What is the admission picker?
+
+The admission picker is used to determine whether the entry should be inserted
+into the disk cache. It is a component of the hybrid cache.
+
+You can use it when you create a hybrid cache. For example:
+
+```rust
+    let hybrid: HybridCache<u64, String> = HybridCacheBuilder::new()
+        .memory(64 * 1024 * 1024)
+        .storage(Engine::Large)
+        .with_admission_picker(Arc::new(RateLimitPicker::new(100 * 1024 * 1024)))
+        .build()
+        .await?;
+```
+
+## Admission types
+
+### RateLimitPicker