Added Insomnia's notes to the knowledge base

knowledge base/insomnia.md

@@ -0,0 +1,22 @@
+# Insomnia <!-- omit in toc -->
+
+## Troubleshooting
+
+### Manually install plugins
+
+1. open the plugins folder in the terminal; get the path in _Preferences_ > _Plugins_ tab > _Reveal Plugins Folder_ button
+1. use `npm` to install the plugin in that folder:
+   ```sh
+   npm i --prefix ./ insomnia-plugin-date-add
+   ```
+
+## Further readings
+
+- [Website]
+- [Postman], an alternative to Insomnia
+
+<!-- internal knowledge base -->
+[postman]: postman.md
+
+<!-- projects' references -->
+[website]: https://insomnia.rest/