Add build artifacts and files to gitignore

Author: SaschaWillems

.gitignore

@@ -9,4 +9,12 @@ temp_ebook.md
 ebook/*.pdf
 ebook/*.epub
 
-convert.py
+convert.py
+
+attachments/build/**
+attachments/android/.gradle/**
+attachments/android/.idea
+attachments/android/.gradle/**
+attachments/android/app/.cxx/**
+attachments/android/app/build/**
+local.properties