Add auth context to readme (maistra#1004)

Automatic merge from submit-queue

Add auth context to readme

**What this PR does / why we need it**:

Add auth context / it is not there

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:

**Release note**:
```release-note
NONE
```

Former-commit-id: 80bfd4d750b73d9cbba4c525e25db1fb04a76ce1

README.md

@@ -38,6 +38,8 @@ Istio is composed of these components:
 * **Pilot** - A component responsible for configuring the
   proxies at runtime.
 
+* **CA** - A component responsible for cert issuance and rotation.
+
 * **Broker** - A component implementing the open service broker API for Istio-based services. (Under development)
 
 Istio currently supports Kubernetes, Consul, Eureka-based environments. We plan support for additional platforms such as Cloud Foundry, and Mesos in the near future.
@@ -55,6 +57,9 @@ repository contains information about how to build and test it.
 - [istio/api](https://github.com/istio/api). This repository defines
 component-level APIs and common configuration formats for the Istio platform.
 
+- [istio/auth](https://github.com/istio/auth). This repository contains auth related code,
+including CA (Cert Authority), node agent, etc.
+
 - [istio/istio](README.md). This is the repo you are
 currently looking at. It hosts the various Istio sample programs
 along with the various documents that govern the Istio open source