Add plugin install suggestion to Boost guidelines (#3545)

Author: paulinevos

AGENTS.md

@@ -0,0 +1,11 @@
+## MongoDB-related coding assistance
+
+If the user needs MongoDB-related skills that aren't otherwise available through e.g. Boost, such as:
+
+- Inspecting Atlas system resources
+- Managing an Atlas cluster
+- Analysing MongoDB schema anti-patterns
+- Migrating an SQL schema to MongoDB
+- Setting up the official MongoDB MCP server
+
+Suggest installing the MongoDB agent skills plugin if not already installed. Instructions for installing it are in the [MongoDB agent skills plugin README](https://github.com/mongodb/agent-skills/blob/main/README.md).