Create BUILD.md

BUILD.md

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+# Build Instructions
+
+This document provides instructions on how to build and work with the Lightning SDK project.
+
+## Prerequisites
+
+Before you begin, ensure you have the following installed on your system:
+
+- [Node.js](https://nodejs.org/) (which includes npm)
+
+## Installation
+
+1.  **Clone the repository:**
+
+    ```bash
+    git clone https://github.com/rdkcentral/Lightning-SDK.git
+    cd Lightning-SDK
+    ```
+
+2.  **Install dependencies:**
+
+    Run the following command in the root directory of the project to install all the necessary dependencies.
+
+    ```bash
+    npm install
+    ```
+
+    This command will also trigger a `postinstall` script that sets up the necessary support files and polyfills in the `support` directory.
+
+## Building
+
+The Lightning SDK is designed to be consumed directly by applications, and as such, it does not have a primary build step that bundles the entire library. The core source files are used as is.
+
+However, the project includes a Rollup configuration to build the `startApp.js` file located in the `support` directory, which is likely used for development or testing purposes.
+
+If you need to manually rebuild this file after making changes to `./src/startApp.js`, you can run:
+
+```bash
+npx rollup -c
+```
+
+This will use the `rollup.config.js` file to transpile and bundle `./src/startApp.js` into `./support/startApp.js`.
+
+## Linting
+
+To check the code for any linting errors, run the following command:
+
+```bash
+npm run lint
+```
+
+## Type Definition Tests
+
+To run the TypeScript definition tests, use the following command:
+
+```bash
+npm run tsd
+```