Add development scripts for smoother development process

Signed-off-by: Egor Savkin <es@m-labs.hk>

README.md

@@ -25,3 +25,6 @@ To build the .bundle.js from .jsx files:
   npm install
   npm run build
   ```
+
+Development builds are also available. `npm run build-dev` to re/build once, or `npm run start-dev` for incremental
+continuous builds as source files change.

package.json

@@ -6,7 +6,9 @@
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1",
     "start": "npx webpack --watch",
-    "build": "npx webpack"
+    "build": "npx webpack",
+    "start-dev": "npx webpack --watch --mode=development --devtool=inline-source-map",
+    "build-dev": "npx webpack --mode=development --devtool=inline-source-map"
   },
   "repository": {
     "type": "git",