docs: readme

Signed-off-by: thxCode <[email protected]>

README.md

@@ -12,6 +12,55 @@ Hermit Crab provides a stable and reliable [Terraform Provider](https://registry
 
 This tool is maintained by [Seal](https://github.com/seal-io).
 
+```mermaid
+sequenceDiagram
+    actor tf  as terraform init
+    participant hc as Hermit Crab
+    participant tfreg as Terraform Registry
+    participant stg as Provider Package Storage
+    
+    tf ->> hc: https://<hermitcrab>/v1/providers/registry.terraform.io/hashicorp/random/index.json
+    alt not found
+        hc ->> tfreg: https://registry.terraform.io/v1/providers/hashicorp/random/versions
+        tfreg -->> hc: 
+        hc ->> hc: save
+        hc -->> tf: 
+    else found
+        hc -->> tf: 
+    end
+    
+    tf ->> hc: https://<hermitcrab>/v1/providers/registry.terraform.io/hashicorp/random/2.0.0.json
+    alt not found
+        par
+            hc ->> tfreg: https://registry.terraform.io/v1/providers/hashicorp/random/versions/2.0.0/download/darwin_amd64
+            tfreg -->> hc: 
+            hc ->> hc: save
+            hc -->> tf: 
+        and
+            hc ->> tfreg: https://registry.terraform.io/v1/providers/hashicorp/random/versions/2.0.0/download/linux_arm64
+            tfreg -->> hc: 
+            hc ->> hc: save
+            hc -->> tf: 
+        end
+    else found
+        hc -->> tf: 
+    end
+    
+    tf ->> hc: https://<hermitcrab>/v1/providers/registry.terraform.io/hashicorp/random/terraform-provider-random_2.0.0_darwin_amd64.zip
+    alt not found
+        hc ->> stg: download https://<storage>/terraform-provider-random_2.0.0_darwin_amd64.zip
+        stg -->> hc: 
+        hc ->> hc: store
+        hc -->> tf: 
+    else not found but downloading
+        hc ->> hc: wait until downloading finished
+        hc -->> tf: 
+    else found
+        hc -->> tf: 
+    end
+
+```
+
 ## Background
 
 When we drive [Terraform](https://www.terraform.io/) at some automation scenarios, like CI, automatic deployment, etc., we need to download the Terraform Provider plugins from the Internet by `terraform init`.