doc update

README.md

@@ -1,11 +1,28 @@
 # LicheeRV-Nano-Build
 
-# how to build
+# download source
 
 ```
 git clone https://github.com/LicheeRV-Nano-Build --depth=1
 cd LicheeRV-Nano-Build
 git clone https://github.com/sophgo/host-tools --depth=1
+```
+
+## host environment
+
+you can use container:
+
+```
+cd host/ubuntu
+docker build -t licheervnano-build-ubuntu .
+docker run --name licheervnano-build-ubuntu
+docker export | sqfstar licheervnano-build-ubuntu.sqfs
+singularity shell -e licheervnano-build-ubuntu.sqfs
+```
+
+# build it
+
+```
 source build/cvisetup.sh
 # C906:
 defconfig sg2002_licheervnano_sd