update readme

Author: TianlongLiang

samples/custom_section/README.md

@@ -2,8 +2,7 @@
 description: "The related code/working directory of this example resides in directory {WAMR_DIR}/samples/custom_section"
 ---
 
-The "custom_section" sample project
-==============
+# The "custom_section" sample project
 
 This sample demonstrates how to:
 
@@ -21,8 +20,20 @@ The Wasm application is built from:
 
 The assembler file emits a section named `.custom_section.demo`, which becomes a Wasm custom section named `demo` in the final `.wasm` file.
 
-Build this sample
-==============
+## Why use a custom section for this payload
+
+The payload in this sample is treated as read-only metadata. Putting it in a custom section lets the embedder access the bytes directly from the loaded module through `wasm_runtime_get_custom_section`, instead of copying the data into Wasm linear memory per instance.
+
+That matters when the data is large or rarely changed:
+
+- the bytes stay in the module image as immutable data
+- the host can look them up by section name and use them in place
+- the Wasm app only needs to pass a small section name and receive a small handle
+- no extra application-level serialization or buffer duplication is needed for the read-only payload
+
+This pattern is useful for embedded assets, lookup tables, model metadata, certificates, and other static blobs that the host wants to consume without treating them as mutable Wasm heap data.
+
+## Build this sample
 
 Execute the `build.sh` script. The host executable and the Wasm app are generated in `out`.
 
@@ -36,8 +47,7 @@ Build the AoT variant only when needed by passing `--aot`. This preserves the `d
 ./build.sh --aot
 ```
 
-Run the sample
-==========================
+## Run the sample
 
 Enter the output directory and run the Wasm sample directly:
 
@@ -57,5 +67,3 @@ To run the AoT artifact instead, pass `--aot` to the helper script:
 ```sh
 ./run.sh --aot
 ```
-
-When running under WSL, use the Linux workspace path layout, for example `/home/tl/projects/wasm-micro-runtime/...`, not a Windows drive path.