Site-www: Add README.md info to package layout page

[kwalrath]

See dart-lang/pub-dartlang-dart#1973 for details.

[kwalrath]

@isoos or @jonasfj, can you point me to a good example of a package that has an example/README.md file? I looked in the sdk repo but didn't find one there.

[isoos]

@kwalrath: Not sure what qualifies as good, but this package has many things in the example directory, and uses a readme.md: https://github.com/flutter/plugins/tree/master/packages/in_app_purchase

[isoos]

A few others:

• https://github.com/jacobaraujo7/bloc-pattern
• https://github.com/DanTup/tiler
• https://github.com/RafaelBarbosatec/achievement_view_flutter
• https://github.com/ganeshsp1/wallpaperplugin

[kwalrath]

Fixed by #1640.