update readme

README.md

 # Nunjucks filters and helpers for generating HTML static pages
 
-## `assetUrl` filter
+## Usage
+
+It exports a single function taking the nunjucks `Environment` instance:
+
+```js
+const env = new nunjucks.Environment(..);
+require('@sidvind/build-nunjucks')(env);
+```
+
+With `grunt-nunjucks-2-html`:
+
+```js
+nunjucks: {
+  options: {
+    configureEnvironment: require('@sidvind/build-nunjucks'),
+  },
+}
+```
+
+## Filters
+
+### `assetUrl` filter
 
 Generate a cache-buster URL for an asset.
 
@@ -16,7 +37,7 @@ Output:
 <link rel="stylesheet" href="assets/css/site.min.css?e4202148fa903327c91d89f0c6fec4af">
 ```
 
-## `sri` filter
+### `sri` filter
 
 SRI integrity (hash) of an asset.
 
@@ -32,11 +53,13 @@ Output:
 <link rel="stylesheet" .. integrity="sha384-zwJH9nUlOkCDg2d1r1c5jLqv8NQ2O9CHKIa0sAYozc0MD1aPSbNqGM9YY7+Sz6n1">
 ```
 
-## `now` global
+## Globals
+
+### `now` global
 
-Build timestamp.
+Build timestamp (seconds since 1 jan 1970).
 
-## `git` global
+### `git` global
 
 Git metadata.