📝 add docset

doc/docset/README.md

@@ -0,0 +1,13 @@
+# docset
+
+The folder contains the required files to create a [docset](https://kapeli.com/docsets) which can be used in
+documentation browsers like [Dash](https://kapeli.com/dash), [Velocity](https://velocity.silverlakesoftware.com), or
+[Zeal](https://zealdocs.org).
+
+The docset can be created with
+
+```sh
+make nlohmann_json.docset
+```
+
+The generated folder `nlohmann_json.docset` can then be opened in the documentation browser.