add substr function to features section

README.md

@@ -14,6 +14,7 @@ wrong (hint: it should be 1!). ES6's `Array.from` tried to solve this, but that
 - Limit string to width (truncate/pad)
 - Unicode-aware string length
 - Unicode-aware substring
+- Unicode-aware substr
 
 🔥 Please note that this library is built for accuracy, not performance. It uses complex regular expressions to
 calculate the string length and perform other operations which are **not** particularly super-jawdropping-fast like
@@ -29,7 +30,7 @@ And import it in your awesome node app:
 ```javascript
 // ES2015+
 import * as stringz from 'stringz'; // OR:
-import { limit, substring, length } from 'stringz';
+import { limit, substring, length, substr } from 'stringz';
 
 // CommonJS
 var stringz = require('stringz');