Added readme for objc examples (Bazel build)

examples/objective-c/README.md

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+# gRPC in 3 minutes (Objective-C with Bazel)
+
+## Background
+There are currently two ways to build projects with the gRPC Objective-C library:
+* Cocoapods & Xcode
+* Bazel
+
+The [helloworld](helloworld) sample is the entry point for the Cocoapods way of building projects. More is documented on grpc.io, including [installation](https://grpc.io/docs/quickstart/objective-c/) and some [tutorials](https://grpc.io/docs/tutorials/basic/objective-c/). They will not be reiterated here and they are not mandatory for the Bazel way.
+
+A good place to get started with Bazel is their official [website](https://bazel.build). And the documentation of using Bazel with gRPC Objective-C library is [here](https://github.com/grpc/proposal/blob/master/L56-objc-bazel-support.md).
+
+## Installation
+To run the examples in Bazel, you should have [Bazel](https://docs.bazel.build/versions/master/install-os-x.html) installed.
+
+## Hello Objective-C gRPC!
+Here's how to build and run the Objective-C implementation of the [Hello World](helloworld) example.
+
+The code for the Hello World example and others live in the `examples` directory. Clone this repository to your local machine by running the following commands:
+```shell
+$ git clone --recursive https://github.com/grpc/grpc
+```
+
+Bazel support for gRPC Objective-C is not yet released, so we need to work on the master branch.
+
+Next, change your directory to `examples/objective-c`
+```shell
+$ cd examples/objective-c
+```
+
+### Try it!
+To run the Hello World sample properly, we need a local server. Let's compile and run the corresponding C++ server:
+```shell
+$ pushd ../cpp/helloworld
+$ make
+$ ./greeter_server &
+$ popd
+```
+Or do it the Bazel way (you might need to open another shell tab for this):
+```shell
+$ bazel run //examples:greeter_server
+```
+
+Now compile the sample:
+```shell
+$ bazel build :HelloWorld
+```
+To run the sample, you need to know the available simulator runtimes in your machine. You could either list the available runtimes yourself:
+```shell
+$ xcrun simctl list
+```
+Or just try running the app and it will let you know what is available:
+```shell
+$ bazel run :HelloWorld
+```
+Note that running this command will build the project even if you forget to build it beforehand.
+
+Finally, launch the app with one of the available runtimes:
+```shell
+$ bazel run :HelloWorld --ios_simulator_version='<runtime>' --ios_sumlator_device='<device>'
+```