added how to for rate limiting (#38)

howto/control-concurrency/Readme.md

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+# Rate limiting an application
+
+A common scenario in distributed computing is to only allow for a given number of requests to execute concurrently.
+Using Dapr, you can control how many requests and events will invoke your application simultaneously.
+
+*Note that this rate limiting is guaranteed for every event that's coming from Dapr, meaning Pub-Sub events, direct invocation from other services, bindings events etc. Dapr can't enforce the concurrency policy on requests that are coming to your app externally.*
+
+## Setting max-concurrency
+
+Without using Dapr, a developer would need to create some sort of a semaphore in the application and take care of acquiring and releasing it.
+Using Dapr, there are no code changes needed to an app.
+
+### Setting max-concurrency in Kubernetes
+
+To set max-concurrency in Kubernetes, add the following annotation to your pod:
+
+<pre>
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: nodesubscriber
+  labels:
+    app: nodesubscriber
+spec:
+  replicas: 1
+  selector:
+    matchLabels:
+      app: nodesubscriber
+  template:
+    metadata:
+      labels:
+        app: nodesubscriber
+      annotations:
+        dapr.io/enabled: "true"
+        dapr.io/id: "nodesubscriber"
+        dapr.io/port: "3000"
+        <b>dapr.io/max-concurrency: "1"<b>
+...
+</pre>
+
+### Setting max-concurrency using the Actions CLI
+
+To set max-concurrency with the Actions CLI for running on your local dev machine, add the `max-concurrency` flag:
+
+`dapr run --max-concurrency 1 --app-port 5000 python ./app.py`.
+
+The above examples will effectively turn your app into a single concurrent service.