Add instructions to quickly test changes in the extractor

- Make use of the local Maven repository

build.gradle

@@ -1,15 +1,23 @@
 allprojects {
     apply plugin: 'java-library'
+    apply plugin: 'maven'
+
     sourceCompatibility = 1.7
     targetCompatibility = 1.7
 
     version 'v0.13.0'
+    group 'com.github.TeamNewPipe'
 
     repositories {
         jcenter()
     }
 }
 
+dependencies {
+    implementation project(':extractor')
+    implementation project(':timeago-parser')
+}
+
 subprojects {
     task sourcesJar(type: Jar, dependsOn: classes) {
         classifier = 'sources'