Added exercise with MongoDB event store single stream projection

workshops/introduction_to_event_sourcing/src/14_projections_single_stream_emmett/README.md

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+# Exercise 14 - Projections
+
+With the selected Emmett's storage, implement the following projections:
+
+1. Detailed view of the shopping cart:
+   - total amount of products in the basket,
+   - total number of products
+   - list of products (e.g. if someone added the same product twice, then we should have one element with the sum).
+2. View with short information about pending shopping carts. It's intended to be used as list view for administration:
+   - total amount of products in the basket,
+   - total number of products
+   - confirmed and canceled shopping carts should not be visible.
+
+Define evolve function in projection definition at the bottom of [projections.exercise.test.ts](./mongodb/projections.exercise.test.ts).
+
+Read more about projections in my article:
+
+- [Guide to Projections and Read Models in Event-Driven Architecture](https://event-driven.io/en/projections_and_read_models_in_event_driven_architecture/?utm_source=event_sourcing_nodejs&utm_campaign=workshop)