docs: add architecture diagram to README (dapr#1549)

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* docs: add architecture diagram showing Java SDK interaction with Dapr runtime (close #<915>)

* docs: add architecture diagram showing Java SDK interaction with Dapr runtime CORRECTION (close #<915>)

* docs: add architecture diagram showing Java SDK interaction with Dapr runtime (close #<915>)

* docs: add architecture diagram showing Java SDK interaction with Dapr runtime (close #<915>)

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Co-authored-by: Siri Varma Vegiraju <siri.varma@outlook.com>
Co-authored-by: artur-ciocanu <artur.ciocanu@gmail.com>
Co-authored-by: Cassie Coyle <cassie.i.coyle@gmail.com>
Signed-off-by: salaboy <Salaboy@gmail.com>

Author: 4 people

README.md

@@ -13,6 +13,25 @@ This is the Dapr SDK for Java, including the following features:
 
 ## Getting Started
 
+### Architecture Overview
+
+The diagram below shows how a Java application interacts with the Dapr runtime through the Java SDK:
+
+```mermaid
+flowchart LR
+    A[End User Application] -->|HTTP or gRPC| B[Dapr Java SDK]
+    B -->|Dapr API calls| C[Dapr Sidecar]
+    C -->|Components| D[State Stores, Pub/Sub, Services, etc.]
+
+    %% Optional grouping for clarity
+    subgraph R[Dapr Runtime]
+        C
+        D
+    end
+```
+
+For the full list of available APIs, see the [Dapr API reference](https://docs.dapr.io/reference/api/)
+
 ### Pre-Requisites
 * SDKMAN! installed (recommended):
     * [SDKMAN!](https://sdkman.io)
@@ -278,4 +297,4 @@ Once the project has been imported, the individual tests can be run normally as
 
 > Sometimes when the `sdk-tests` project does not build correctly, try `File > Invalidate Caches...`  and try restarting IntelliJ.
 
-You should be able to set breakpoints and Debug the test directly from IntelliJ itself as seen from the above image. 
+You should be able to set breakpoints and Debug the test directly from IntelliJ itself as seen from the above image.