Add AGENTS.md (#1264)

This file is read by LLM agents when working on the codebase:
https://agentsmd.net/

With this file you don't need to copy paste these instructions in every
prompt.

AGENTS.md

@@ -0,0 +1,43 @@
+Maputnik is a MapLibre style editor written using React and TypeScript.
+
+To get started, install all npm packages:
+
+```
+npm install
+```
+
+Verify code correctness by running ESLint:
+
+```
+npm run lint
+```
+
+Or try fixing lint issues with:
+
+```
+npm run lint -- --fix
+```
+
+The project type checked and built with:
+
+```
+npm run build
+```
+
+To run the tests make sure that xvfb is installed:
+
+```
+apt run xvfb
+```
+
+Run the development server in the background with Vite:
+
+```
+nohup npm run start &
+```
+
+Then start the Cypress tests with:
+
+```
+xvfb-run -a npm run test
+```