adding readme for sumofsq appdeploy sample.

Author: jeroenterheerdt

samples/features/sql-big-data-cluster/app-deploy/sumofsq/README.md

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+# Running a basic R script in SQL Server big data cluster
+
+### Contents
+
+[About this sample](#about-this-sample)<br/>
+[Before you begin](#before-you-begin)<br/>
+[Run this sample](#run-this-sample)<br/>
+[Sample details](#sample-details)<br/>
+[Related links](#related-links)<br/>
+
+<a name=about-this-sample></a>
+
+## About this sample
+
+This is a sample [R](https://www.r-project.org/) app, which takes two whole numbers as input and returns the sum of squares (a^2 + b^2). The code for this sample is in [sum_of_squares.R](sum_of_squares.R) The inputs and outputs are shown below.
+
+### Inputs
+|Parameter|Description|
+|-|-|
+|`a`|The first whole number|
+|`b`|The second whole number|
+
+### Outputs
+|Parameter|Description|
+|-|-|
+|`result`|The result of calculating a^2 + b^2|
+
+
+<a name=before-you-begin></a>
+
+## Before you begin
+
+To run this sample, you need the following prerequisites.
+
+**Software prerequisites:**
+
+1. SQL Server big data cluster CTP 2.3 or later.
+2. `mssqlctl`. Refer to [installing mssqlctl](https://docs.microsoft.com/en-us/sql/big-data-cluster/deploy-install-mssqlctl?view=sqlallproducts-allversions) document on setting up the `mssqlctl` and connecting to a SQL Server 2019 big data cluster.
+
+<a name=run-this-sample></a>
+
+## Run this sample
+
+1. Clone or download this sample on your computer.
+2. Log in to the SQL Server big data cluster using the command below using the IP address of the `endpoint-service-proxy` in your cluster. If you are not familiar with `mssqltctl` you can refer to the [documentation](https://docs.microsoft.com/en-us/sql/big-data-cluster/big-data-cluster-create-apps?view=sqlallproducts-allversions) and then return to this sample.
+
+    ```bash
+    mssqlctl login -e https://<ip-address-of-endpoint-service-proxy>:30777 -u <user-name> -p <password>
+    ```
+3. Deploy the application by running the following command, specifying the folder where your `spec.yaml` and `sum_of_squares.R` files are located:
+    ```bash
+    mssqlctl app create --spec ./sumofsq
+    ```
+4. Check the deployment by running the following command:
+    ```bash
+    mssqlctl app list -n sum-of-sq -v [version]
+    ```
+    Once the app is listed as `Ready` you can continue to the next step.
+5. Test the app by running the following command:
+    ```bash
+    mssqlctl app run -n sum-of-sq -v [version] --input a=42,b=3435
+    ```
+    You should get output like the example below. The answer to your question are returned as `result`.
+    ```json
+    {
+      "changedFiles": [],
+      "consoleOutput": "",
+      "errorMessage": "",
+      "outputFiles": {},
+      "outputParameters": {
+        "result": 11800989.0
+      },
+      "success": true
+    }
+    ```
+6. You can clean up the sample by running the following commands:
+    ```bash
+    # delete app
+    mssqlctl app delete --name sum-of-sq --version [version]
+    ```
+
+<a name=sample-details></a>
+
+## Sample details
+
+Please refer to [sum_of_squares.R](sum_of_squares.R) for the code for this sample.
+
+### Spec file
+Here is the spec file for this application. As you can see the sample uses the `R` runtime and calls the `sofsq` method in the `sum_of_squares.R` file, accepting two integer inputs named `a` and `b` and returning an integer output named `result`.
+
+```yaml
+name: sum-of-sq
+version: v1
+runtime: R
+src: ./sum_of_squares.R
+entrypoint: sofsq
+replicas: 1
+poolsize: 1
+inputs:
+  a: integer
+  b: integer
+output:
+  result: integer
+```
+
+<a name=related-links></a>
+
+## Related Links
+For more information, see these articles:
+
+[How to deploy and app on SQL Server 2019 big data cluster (preview)](https://docs.microsoft.com/en-us/sql/big-data-cluster/big-data-cluster-create-apps?view=sqlallproducts-allversions)