Add npm package section to readme

README.md

···
 
 An example page that can decode Base64-encoded (raw base64, PEM armoring and `begin-base64` are recognized) or Hex-encoded (or local files with some browsers) is included and can be used both [online on the official website](https://lapo.it/asn1js/) or [offline (ZIP file)](https://lapo.it/asn1js/asn1js.zip).
 
+Usage with npm / yarn
+--------------
+
+This package can be installed with either npm or yarn via the following commands:
+
+```
+npm install @lapo/asn1js
+# or with yarn
+yarn add @lapo/asn1js
+```
+
+Assuming a standard javascript bundler is setup you can import it like so:
+```js
+const asn1js = require('@lapo/asn1js');
+// or with ES modules
+import asn1js from '@lapo/asn1js';
+```
+
+A submodule of this package can also be imported:
+```js
+import Hex from '@lapo/asn1js/hex';
+```
+
 ISC license
 -----------