add atom s3r build guide

main/boards/atoms3r-echo-base/README.md

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+# 编译配置命令
+
+**配置编译目标为 ESP32S3：**
+
+```bash
+idf.py set-target esp32s3
+```
+
+**打开 menuconfig：**
+
+```bash
+idf.py menuconfig
+```
+
+**选择板子：**
+
+```
+Xiaozhi Assistant -> Board Type -> AtomS3R + Echo Base
+```
+
+**修改 flash 大小：**
+
+```
+Serial flasher config -> Flash size -> 8 MB
+```
+
+**修改分区表：**
+
+```
+Partition Table -> Custom partition CSV file -> partitions_8M.csv
+```
+
+**修改 psram 配置：**
+
+```
+Component config -> ESP PSRAM -> SPI RAM config -> Mode (QUAD/OCT) -> Octal Mode PSRAM
+```
+
+**编译：**
+
+```bash
+idf.py build
+```