sdio_slave: add some documentation about impl in the source code

components/driver/sdio_slave/README.md

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+# SDIO workflow
+
+This README is a supplement to the comments in the SDIO slave driver. For more details, see `sdio_slave.c`.
+
+The diagram are described in mermaid format. You can view the diagrams on Github or with any mermaid renders.
+
+## Sending direction
+
+```mermaid
+sequenceDiagram
+    actor task
+    participant ISR
+    participant HW
+    Note right of HW: always: rx_done.raw = 1, rx_eof.ena= 1
+
+    task-->>ISR: Prepare transfers
+    task->>HW: rx_done.ena=1
+    activate HW
+
+    %% First ISR triggering transfer
+    HW->>ISR: trigger
+    activate ISR
+
+    ISR->>HW: rx_done.ena=0
+    deactivate HW
+
+    loop every finished descriptor
+    note over task, HW: skipped, see below
+    end
+
+    opt HW idle && new transfer prepared
+    ISR->>HW: Load transfer
+    end
+    deactivate ISR
+
+    activate HW
+    HW->>HW: Host doing RX transfer
+    deactivate HW
+
+    HW->>HW: rx_eof.raw=1
+    activate HW
+
+    %% Second ISR recycling transfer
+    HW->>ISR: trigger
+    activate ISR
+
+    ISR-->>HW: rx_done.ena=0
+
+    loop every finished descriptor
+    ISR->>HW: rx_eof.raw=0
+    deactivate HW
+    ISR-->>task: inform
+    end
+
+    opt HW idle && new transfer prepared
+    note over ISR, HW: Load next transfer if there is...
+    end
+
+    deactivate ISR
+```