Added contribution guideline.
Fix that the scroll position of ObservableListView is not restored after rotating the screen.
Fix that the scroll position of ObservableScrollView is not restored after rotating the screen.
Fix that the scroll position of ObservableRecyclerView is not restored after rotating the screen.
Fix that the scroll position of ObservableWebView is not restored after rotating the screen.
Fix that the scroll position of ObservableGridView is not restored after rotating the screen.
Fix that ObservableRecyclerView's scrollY is incorrect when scrolling from the bottom.
This is not the perfect solution and not completely fixed the issue, but now it's better.
Add example of ObservableRecyclerView scrolling from bottom.

Note that this example has a problem that it cannot hide toolbar around item 1 and 2.
This is just a sample to confirm the result of the fix in a99a0de.