Add build instructions

Makefile

+all: prod
+
+prod:
+	./gradlew jsBrowserProductionWebpack
+	rm -f build/kemence.zip
+	bash -c "cd build/distributions && rm -f *.js.map && zip -r ../kemence.zip ."
+
+firefox:
+	rm -f build/firefox.zip
+	zip -r build/firefox.zip \
+		src \
+  	 	gradle \
+  	 	build.gradle.kts \
+  	 	settings.gradle.kts \
+  	 	README.md \
+  	 	Makefile \
+  	 	gradlew \
+  	 	gradlew.bat \
+  	 	gradle.properties
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README.md

+# Kemence
+an extension to improve PéK
+
+## How to build
+A working install of JDK is needed to build the extension.
+All other dependencies are managed by Gradle.
+
+```sh
+# prod build
+./gradlew jsBrowserProductionWebpack
+cd build/distributions
+
+# dev build
+./gradlew jsBrowserDevelopmentWebpack
+cd build/developmentExecutable
+```
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