docs(replace): clarify options for allowed replacement values (#422)

packages/replace/README.md

@@ -36,13 +36,17 @@ export default {
     dir: 'output',
     format: 'cjs'
   },
-  plugins: [replace({ __buildEnv__: 'production' })]
+  plugins: [replace({ 
+    __buildEnv__: 'production',
+    __buildDate__: () => new Date(),
+    __buildVersion: 15
+  })]
 };
 ```
 
 Then call `rollup` either via the [CLI](https://www.rollupjs.org/guide/en/#command-line-reference) or the [API](https://www.rollupjs.org/guide/en/#javascript-api).
 
-The configuration above will replace every instance of `__buildEnv__` with `'production'` in any file included in the build. _Note: Values should always be strings. For complex values, use `JSON.stringify`._
+The configuration above will replace every instance of `__buildEnv__` with `'production'` and `__buildDate__` with the result of the given function in any file included in the build. _Note: Values have to be either primitives or functions that return a string. For complex values, use `JSON.stringify`._
 
 Typically, `@rollup/plugin-replace` should be placed in `plugins` _before_ other plugins so that they may apply optimizations, such as dead code removal.