#192: Add BUILD.md

Author: mplieske

BUILD.md

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+# Build
+
+The following instructions explain how to build KeepTime from sources.
+
+## Prerequisites
+
+* JDK with JavaFx
+  * 17 ✅
+  * 21 ✅
+  * 23 ✅
+  * 25 ❗ (Compatible for building without test)
+* Maven
+
+Amazon Corretto JDK has JavaFx Bundled.  
+For other JDKs you may have to install JavaFx separately.
+
+The Mockito mocking library version in use for tests is not compatible with Java 25.  
+If you trust us and you don't need to run test but you just want to build from the stable `main` branch, Java 25 is supported.
+
+## Build Commands
+
+Run one of the following commands from the root directory of this project, where the `pom.xml` file is found.
+
+A zip file and a jar with dependencies bundled with it will be created at the `target` directory.  
+If you don't use the profile `coverage`, crutial parts of the application will not be built.
+
+### Build Without Tests
+
+`mvn package -P coverage -D maven.test.skip true`
+
+Only recommended on the stable `main` branch.
+
+### Build With Tests
+
+`mvn package -P coverage`
+
+### Build With Coverage Reports
+
+`mvn package org.jacoco:jacoco-maven-plugin:0.8.7:prepare-agent org.jacoco:jacoco-maven-plugin:0.8.7:report -P coverage`