updated docs

package.json

@@ -3,7 +3,7 @@
   "version": "1.0.0",
   "private": true,
   "scripts": {
-    "build": "PUBLIC_URL='./' node scripts/build.js",
+    "build": "cross-env PUBLIC_URL='./' node scripts/build.js",
     "start": "node scripts/start.js",
     "test": "node scripts/test.js"
   },
@@ -138,6 +138,7 @@
     "bundle-loader": "0.5.6",
     "case-sensitive-paths-webpack-plugin": "2.2.0",
     "chalk": "2.4.2",
+    "cross-env": "^7.0.3",
     "css-loader": "2.1.1",
     "dotenv": "8.0.0",
     "dotenv-expand": "5.1.0",

yarn.lock

@@ -3551,6 +3551,13 @@ cross-env@^6.0.3:
   dependencies:
     cross-spawn "^7.0.0"
 
+cross-env@^7.0.3:
+  version "7.0.3"
+  resolved "https://registry.yarnpkg.com/cross-env/-/cross-env-7.0.3.tgz#865264b29677dc015ba8418918965dd232fc54cf"
+  integrity sha512-+/HKd6EgcQCJGh2PSjZuUitQBQynKor4wrFbRg4DtAgS1aWO+gU52xpH7M9ScGgXSYmAVS9bIJ8EzuaGw0oNAw==
+  dependencies:
+    cross-spawn "^7.0.1"
+
 [email protected], cross-spawn@^6.0.0, cross-spawn@^6.0.5:
   version "6.0.5"
   resolved "https://registry.yarnpkg.com/cross-spawn/-/cross-spawn-6.0.5.tgz#4a5ec7c64dfae22c3a14124dbacdee846d80cbc4"
@@ -3588,6 +3595,15 @@ cross-spawn@^7.0.0:
     shebang-command "^2.0.0"
     which "^2.0.1"
 
+cross-spawn@^7.0.1:
+  version "7.0.3"
+  resolved "https://registry.yarnpkg.com/cross-spawn/-/cross-spawn-7.0.3.tgz#f73a85b9d5d41d045551c177e2882d4ac85728a6"
+  integrity sha512-iRDPJKUPVEND7dHPO8rkbOnPpyDygcDFtWjpeWNCgy8WP2rXcxXL8TskReQl6OrB2G7+UJrags1q15Fudc7G6w==
+  dependencies:
+    path-key "^3.1.0"
+    shebang-command "^2.0.0"
+    which "^2.0.1"
+
 crypto-browserify@^3.11.0:
   version "3.12.0"
   resolved "https://registry.yarnpkg.com/crypto-browserify/-/crypto-browserify-3.12.0.tgz#396cf9f3137f03e4b8e532c58f698254e00f80ec"