[doc]: add build note to use normal user account to do the make (#1393)

* [doc]: add build note to use normal user account to do the make

Author: lguohan

README.md

@@ -52,8 +52,9 @@ To build SONiC installer image and docker images, run the following commands:
 
  **NOTE**:
 
-- We recommend reserving 50G free space to build one platform.
-- Use ```http_proxy=[your_proxy] https_proxy=[your_proxy] make``` to enable http(s) proxy in the build process.
+- Recommend reserving 50G free space to build one platform.
+- Use `http_proxy=[your_proxy] https_proxy=[your_proxy] make` to enable http(s) proxy in the build process.
+- Add your user account to `docker` group and use your user account to make. `root` or `sudo` are not supported.
 
 The SONiC installer contains all docker images needed. SONiC uses one image for all devices of a same ASIC vendor. The supported ASIC vendors are: