Add bin/ folder to .gitignore

Previously, running the command ./gradlew in the terminal
generated a bin folder containing all the .class files.
To prevent accidentally committing files to the repository,
let's adds /bin/ to the .gitignore file

.gitignore

@@ -1,6 +1,7 @@
 # Gradle build files
 /.gradle/
 /build/
+/bin/
 src/main/resources/docs/
 
 # IDEA files