Add note for XML convention

CONTRIBUTING.md

@@ -0,0 +1,17 @@
+# Contribution Guidelines
+
+## Architecture
+
+TBD
+
+## Coding convention
+
+## Kotlin
+## XML
+
+Please sort the attributes as follow:
+
+- ID of a View is always on top, unless the namespace is defined, in which case ID comes right after the namespace definition.
+- Right after the ID is its content, if applicable (e.g. `android:text` for `TextView`, `android:src` for `ImageView`).
+- Event listeners (e.g. `android:onClick`) are put at the end.
+- All other attributes are sorted alphabetially.