Support custom assertion messages (#204)

Co-authored-by: Sindre Sorhus <[email protected]>
sindresorhus · Apr 23, 2024 · f2e5834 · f2e5834
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diff --git a/readme.md b/readme.md
@@ -45,6 +45,15 @@ assert.string(2);
 //=> Error: Expected value which is `string`, received value of type `number`.
 ```
 
+Assertions (except `assertAll` and `assertAny`) also support an optional custom error message.
+
+```js
+import {assert} from '@sindresorhus/is';
+
+assert.nonEmptyString(process.env.API_URL, 'The API_URL environment variable is required.');
+//=> Error: The API_URL environment variable is required.
+```
+
 And with TypeScript:
 
 ```ts