Add svgo as npm dependency and set up npm script to run it

.gitignore

@@ -0,0 +1,3 @@
+# Don't track the Node.js installation artifacts
+/node_modules/
+/package-lock.json

README.md

@@ -55,14 +55,25 @@ npm install --save https://github.com/HatScripts/circle-flags
 
 ## Contributing
 
-To contribute, you need to have the latest version of [svgo](https://github.com/svg/svgo) installed.
+### Initial setup
+
+To contribute, you need to have the [Node.js](https://nodejs.org) JavaScript runtime installed,
+and the latest version of [svgo](https://github.com/svg/svgo).
+Once Node.js is set up, you can run the following command to install `svgo` in the circle-flags project directory:
+
+```sh
+npm install
+```
+
+### Making changes
 
 First, edit the relevant SVG files in the `flags/` directory.
 
-Then run `svgo` to optimize the SVG files:
+Then run the `svgo` wrapper script (defined in the `scripts` section of [package.json](./package.json)),
+which uses the locally-installed `svgo` executable to optimize the SVG files:
 
 ```sh
-svgo ./flags --recursive --config=svgo.config.js
+npm run svgo
 ```
 
 Then commit the changes, and submit them as a pull request.

package.json

@@ -33,5 +33,11 @@
   "bugs": {
     "url": "https://github.com/HatScripts/circle-flags/issues"
   },
-  "homepage": "https://github.com/HatScripts/circle-flags#readme"
+  "homepage": "https://github.com/HatScripts/circle-flags#readme",
+  "devDependencies": {
+    "svgo": "^3.0.2"
+  },
+  "scripts": {
+    "svgo": "node_modules/.bin/svgo ./flags --recursive --config=svgo.config.js"
+  }
 }