Adds custom assertion message example to README

readme.md

@@ -45,6 +45,15 @@ assert.string(2);
 //=> Error: Expected value which is `string`, received value of type `number`.
 ```
 
+Most assertions also support an optional custom error message.
+
+```js
+import {assert} from '@sindresorhus/is';
+
+assert.nonEmptyString(process.env.API_URL, 'API_URL env variable is required.');
+//=> Error: API_URL env variable is required.
+```
+
 And with TypeScript:
 
 ```ts