Add build instructions

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BUILD.md

		+# Introduction
		+
		+Build instructions for the application.
		+
		+# Requirements
		+
		+Install the following:
		+
		+* [Gradle](https://gradle.org) version 6.7 or greater.
		+* [Git](https://git-scm.com) version 2.31.1 or greater.
		+
		+Ensure both Gradle and Git may be run from the command-line. This may
		+entail setting the `PATH` environment variable.
		+
		+# Build
		+
		+Build the application as follows:
		+
		+1. Open a terminal.
		+1. git clone https://github.com/DaveJarvis/kmcaster
		+1. cd kmcaster
		+1. gradle clean build
		+
		+The application is built.
		+