docs(README): clarify async usage (pattern.transform)

[adgllorente]

Issues

• Fixes [Docs] Async transform #160

Edit transform documentation to specify that a Promise may be returned.

[jsf-clabot]

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[michael-ciniawsky]

@adgllorente Thx in advance :)

[michael-ciniawsky]

|[`transform`](#transform)|`{Function\|Promise}`|`(content, path) => content`|Function or Promise that modifies file contents before copying|

[michael-ciniawsky]

Adding an example for {Promise} would be highly appreciated (~L201+), but not mandatory :)

 ### `transform`
 
+ #### `{Function}`

 **webpack.config.js**
 ```js
 [
   new CopyWebpackPlugin([
     {
       from: 'src/*.png',
       to: 'dest/',
       transform (content, path) {
         return optimize(content)
       }
     }
   ], options)
 ]

+ #### `{Promise}`

+  **webpack.config.js**
+  ```js
+  [
+    new CopyWebpackPlugin([
+     {
+       from: 'src/*.png',
+       to: 'dest/',
+       transform (content, path) {
+         return Promise.resolve(optimize(content))
+       }
+    }
+   ], options)
+ ]

[michael-ciniawsky]

@adgllorente Thx