README: re-add Go version note

This reverts commit f4366abc71c169a16893fa2cfbfa48453233f5b9.

Signed-off-by: Tianling Shen <[email protected]>

README.md

@@ -70,3 +70,15 @@
   ./scripts/feeds install -a
   ```
 
+### Note
+
+#### ⚠ For OpenWrt 21.02 or lower version
+You have to manually upgrade Golang toolchain to [1.19](https://github.com/openwrt/packages/tree/openwrt-22.03/lang/golang) or higher to compile Xray-core.
+
+e.g.:
+
+```bash
+./scripts/feeds update packages
+rm -rf feeds/packages/lang/golang
+svn co https://github.com/openwrt/packages/branches/openwrt-22.03/lang/golang feeds/packages/lang/golang
+```