add README.md on how to run examples for effect-mongodb package

Author: devmatteini

packages/effect-mongodb/examples/README.md

@@ -0,0 +1,16 @@
+# effect-mongodb Examples
+
+This directory contains some examples of `effect-mongodb` package.
+
+In root directory of this repository you can find a `docker-compose.yml` that sets up a local MongoDB instance.
+
+```shell
+pnpm run infra
+```
+
+Then you can run the examples with:
+
+```shell
+cd packages/effect-mongodb
+pnpm tsx examples/<example>.ts
+```