Improve docs for customisations

The new regex considers Windows as an OS, and the added array helps reduce merge conflicts for the future.

docs/customisations.md

@@ -17,11 +17,11 @@ the React SDK, you can still override it from the Element Web layer:
 2. Edit customisations points and make sure export the ones you actually want to
    activate
 3. Tweak the Element build process to use the customised module instead of the
-   default by adding this to end of the `plugins` array in `webpack.config.js`:
+   default by adding this to the `additionalPlugins` array in `webpack.config.js`:
 
 ```js
 new webpack.NormalModuleReplacementPlugin(
-    /src\/customisations\/Security.ts/,
+    /src[\/\\]customisations[\/\\]Security\.ts/,
     path.resolve(__dirname, 'src/customisations/YourNameSecurity.ts'),
 ),
 ```

webpack.config.js

@@ -8,6 +8,10 @@ const webpack = require("webpack");
 let og_image_url = process.env.RIOT_OG_IMAGE_URL;
 if (!og_image_url) og_image_url = 'https://app.element.io/themes/element/img/logos/opengraph.png';
 
+const additionalPlugins = [
+    // This is where you can put your customisation replacements.
+];
+
 module.exports = (env, argv) => {
     if (process.env.CI_PACKAGE) {
         // Don't run minification for CI builds (this is only set for runs on develop)
@@ -357,6 +361,8 @@ module.exports = (env, argv) => {
                 minify: argv.mode === 'production',
                 chunks: ['usercontent'],
             }),
+
+            ...additionalPlugins,
         ],
 
         output: {