better `vivadow init`

vivado-wrapper

@@ -154,10 +154,12 @@ function clean_real_project () {
 }
 
 function do_init () {
-    mkdir constraint build src
+    mkdir -p constraint build src
     [[ -f ./Vivadofile ]] && mv ./Vivadofile ./Vivadofile.backup
     cp "$my_path"/template/Vivadofile ./Vivadofile
-    echo "init done."
+    echo "I'll provide a constraint file for xc7a100tcsg324-1, which is used by HUST students. Remove it if it's not your case."
+    cp "$my_path"/template/xc7a100tcsg324-1.xdc ./constraint
+    echo "Vivadow project inited."
 }
 
 function do_build () {