Adds an outline to the spinner

Some browsers paint the select box with no borders and looks weird, so this
adds and outline to the component (it does not add a border because that would
override the green border painted when a component is selected).

Change-Id: I121a9bd98da8f78fb548532912ea16e9bc2bb6ee
Adds some contributor guidelines to README

Change-Id: I10ddf3e37f85a4af36c9b4575806b91400f3064f
Merge branch 'master' of https://github.com/mit-cml/appinventor-sources into menu
Label format was correct, new versions added for label and YOUNG_ANDROID_VERSION