docs: add Deno reducer example (#156)

book/src/SUMMARY.md

@@ -1,3 +1,5 @@
 # Summary
 
 - [Introduction](./introduction.md)
+- [Reducers](./reducers/README.md)
+    - [Deno](./reducers/deno.md)

book/src/reducers/README.md

@@ -0,0 +1,3 @@
+# Reducers
+
+- [Deno](deno.md): custom reducers in js

book/src/reducers/deno.md

@@ -0,0 +1,29 @@
+# Deno
+
+With the deno reducer is possible to create a custom reducer for logics that the builtin reducers don't have support. This reducer is only enable with the deno feature on build.
+
+## Configuration
+
+Example of a configuration
+
+```toml
+[reducer]
+type = "Deno"
+main_module = "./examples/deno/enrich.js"
+use_async = true
+```
+
+### Section: `reducer`
+
+- `type`: the literal value `Deno`.
+- `main_module`: the js file with the reducer logic
+- `use_async`: run the js in async mode
+
+
+## Run code
+
+To run the code with the deno will be necessary to use deno feature
+
+```sh
+cargo run --features=deno -- daemon --config ./examples/deno/enrich.js
+```